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FRIDA1 quick tutorial

Frida1 quick tutorial
Introduction

Frida1 is a nice data treatment software to deal with neutrons data mainly, and this page is about how to work with it, hands to dough. A new version is also available here

This tutorial aims to show typical workflows. It is not thought to be thoughtlessly copied but rather to show how some of the commands in Frida-1 can be used. Hopefully, one feels eager to further explore this powerful program...

Some Basic Relations in Neutron Scattering

That is not an extensive document about all the possible relationships in neutron scattering, the relationships are written here, as the users need them...

  • From meV to frequency: 1meV=240GHz
  • Resolution function with = maximum accesible time τ(ps)=0.658/Res(meV)
  • Wavelength of Thermal Neutrons: 1.8Å
  • Bose Factor 1/(exp(h·f/kBT)-1). If h·f=E(meV), then the Boltzmann constant kB = 0.08617373 meV/K
  • If you get ω from the quasielastic line, plotted as a function of Q2, the slope is the diffusion coeficient in meV·Å2. To get it in 10-4 cm2/s, multiply it by 1.519
  • To calculate the susceptibility from s(q,w): chi’‘=s(q,w)*hw/(KbT)
  • Niquist Shannon: If you have S(q,w) in mev, the minimum time you can observe (because of NS sampling theorem) is Δt=0.658*π/(Emax).


    Self Absorption Correction

    First the good news: if you have a sample whose transmission is bigger than 0.9 (of course 0.9 is some kind of arbitrary limit!!!), the shading of the walls by the sample is minor, and you can say in a first approximation that your signal is that of your sample plus that of your container.

    Now the bad news: if your sample has a transmission smaller than 0.9, there will be more neutrons that will “see” the first wall they found than the last wall of the container, because the sample has decreased the number of neutrons going through. The signal of your container is overestimated in the measurement of the pure empty can compared to the measurement of the sample+container. This overestimation depends on the scattering angle. One way to correct that is to use the ideas of Paalman and Pings wrote down in a classical paper (see J. App. Phys. 33 (8)2635, from 1962, when was usual not to perform simulations, but work out nice and long formulas ;-)). This is implemented in IDA by the formula (see also Bee p 142 where the notation is taken from):

    Ssample(2θ,ω)=1/Assc * (Ssample+container(2θ,ω)-Arel*Scontainer(2θ,ω))

    IDA will give you the 2θ dependent values of Assc and Arel. The first one corrects the absolute intensity, and the second the relative contribution of your container related to that of your sample, therefore if you get a number bigger than 1, you made something wrong... In case you set the transmission of the inner and outer can to 1 (which is the case for “normal” Al-cans at large wavelengths), Assc=Arel – in this case, Arel is not calculated by the routine. Now let’s put hands into dough (Spanish expression ;-). Let’s do the case of a cylindrical sample. We start with file 1, where your data are (no prior background subtraction):

    1 you’re in your data file
    _sac for Self Absorption Correction
    3 hollow cylinder
    2.26 The outer radius of your sample (that is the inner radius of your outer sample container) in cm
    0.05 The thickness of your sample layer
    0.98 Transmission of the inner container.
    0.05 Thickness of the inner container
    0.98 Transmission of the outer container
    0.05 Thickness of outer container
    0 Fraction of backscattering traversing the sample: for backscattering machines only, where the neutrons go two times through the sample
    4.641e-3 Number density of scatterers for of your sample, which is the density(g/cc)*602*#atoms/Molecular weight (602 comes from Avogadro’s number
    96928 Scattering cross section, which can be calculated from tables at: www.ncnr.nist.gov/resources/n-lengths/ . By the way, here do not divide by the number of atoms in your molecule, it has already being done in the calculation of density
    746.33 Absorption cross section, also in the aforementioned web
    y it is all right!!!
    80 Angular meshes
    80 Radial meshes. Some explanations about the two last values: to do the calculations the program will make a grid of angles and radius. 80 seems to be a reasonable value for both, but it depends on the case. However the bigger the values, the longer it takes!

    You should now have two files with Assc, and Arel. Now we perform the calculations to obtain the corrected intensity, and now:

    First step : dividing the S sample + container file by the *.Assc file 1) select the S sample + container file 2) type : oy (for operation on y) 3) type : / or 55 4) 2nd argument ? y2 5) choose the *.Assc file

    Then you have the S sample + container / Assc

    Second step : dividing the *.Arel file by the *.Assc file : 1) select the *.Arel 2) type: oy (for operation on y) 3) type : / or 55 4) 2nd argument ? y2 5) choose the *.Assc file

    Then I have Arel / Assc

    Third step : multiplying the S container file by the (Arel / Assc) file created in the second step : 1) select the S container file 2) type : oy (for operation on y) 3) type : * or 54 4) 2nd argument ? y2 5) choose the file created in the second step

    Then we have S container * (Arel / Assc)

    Fourth step : subtraction of the (S sample + container / Assc) file from the S container * (Arel / Assc) file : 1) select S sample + container / Assc file 2) type : oy 3) type : – or 52 4) 2nd argument ? y2 5) choose (S sample + container / Assc)
    Then we have the S sample file after subtraction of the empty can with self absorption correction.

    Self absorption correction of vanadium

    We will perform the SAC for Vanadium, in order to divide by the right Q dependence our data... In file 1 is the vanadium, read WITHOUT vanadium numors.

    1 _sac
    0 fully iluminated
    3 hollow cylinder
    1.17 outer radius of the sample, in this case vanadium without container from TOFTOF
    0.06 thickness of vanadium (the TOFTOF one in this case)
    1 The transmission are set to 1 because there is no sample holder
    1 again
    0 no backscattering
    72 the number density for vanadium
    5.08 scattering cross section of V
    5.08 absorption xs
    80
    80 seems reasonable

    And now you should get ONLY Assc, since there is no sample holder, there will be no Arel. And now:

    1 where our vanadium is
    oy
    /
    y2
    2 where our Assc is
    3 our divided data
    oi
    4 integrate
    -0.1 0.1 this should contain the elastic peak. extract the HWHM before (another oi option) and integrate from -2*HWHM to 2*HWHM.
    1 to save as a file

    And now we have got the “numbers” to normalize your sample!!! Before going on, you should however check that the integral performed with and without SAC correction are not similar. If they are similar, maybe is not worth to do it for every sample... up to you!

     

    Read data from TOFTOF after convert:

    Some words before the description of the procedure. Vanadium data are used to:

    • Search for the elastic channel. This is done taking the maximum of Vanadium spectra, and assigning the origin to this channel
    • Normalize of the intensity as a function of the angle (detector). That means that the program will integrate the peak of Vanadium (from the maximum, to 2*(half width at half maximum) in both directions of the peak) for every spectrum, will get a value, and afterwards it will divide each spectrum of your sample to this value. This has the problem that vanadium is NOT corrected for self absorption, and can give a slightly wrong angle dependence. You can then correct your vanadium using the self absorption correction procedure. It’s also explained below.
    • The detector efficiency correction, is energy dependent and has nothing to do with Vanadium. It simply takes in to account the efficiency of detector that would be less as the energy of the neutron increases, and is implemented as a formula.
    rtof to read TOF data in a pseudo-IN6 format
    0 read current cycle data
    0 number offset
    13940-13943,13945 sample numors
    13333-13334 Background numors ( write - if you have no empty can measurement) or want to subtract it later on after self absorption correction
    1222-1223 Vanadium numors ( - if you do not have it) used for normalization of the detected intensity. is not working correctly only in the TOFTOF ida version, therefore do not trust the intensities when not using that one.
    - Vanadium Background numors
    y to normalize to monitor (appears only if no vanadium numors were given). Works only correctly in the TOFTOF ida version (wavelength-dependence of monitor sensitivity was not accounted for)
    y to convert to energy
    n generate sig(2th,w): say y if you want to obtain d2σ/(dΩ dE) instead of S(2θ,E) – only difference is a factor kf/ki
    0 elastic channel from vanadium. it will search the channel with highest intensity for every detector. do not take elastic channel from sample because this search is done in time space. the subsequent multiplication (when going to E space) with t4 shifts any broadened peak.
    y to correct detector efficiency
    y to take Debye Waller Factor from Vanadium. if measured with a non-standard temperature, frida will ask for confirmation of that temperature later on. check the DWF in that case.
    4 to treat frame overlap at TOFTOF
    1000 maximal energy gain (will scroll data)
    0.99 maximal energy loss (will delete data)
    n do not sum spectra at equal angle so that bad detectors are not masked by taking the average with a good one
    1-3 to exclude spectra which contain monitor informations (they are not all used but are in the converted file for consistency with IN6)
    3 to delete spectra if vanadium data are bad
    - to go out from the read menu
    df the program will write the files it has (analog to “ls”)

    You can now perform the self absortion correction, explained here.


    Fast Fourier Transforamtion of Data

    Here we will shortly explain how to Furier transform the S(Q,ω) to I(Q,t) by the Fast Fourier Transformation algorithm implemented in FRIDA1. Please do take into account a couple of things:

    • The space between points in frequency space gives an upper limit in the time space, i.e., the upper limit of times will be given roughly by tmax=2*pi/Δtmin
    • On the other hand the difference between the last and first frequencies will give you the lower limit in time space, given again by the same formula than before tmin=2*pi/Δtmax.

    Therefore do not cut your S(Q,ω) and try to rebin your data with the lowest spacing (in w) between points (be reasonable however!!!! do not bin the data in a grid smaller than that of the experiment. That will not help at all ;-)).

    We will explain, therefore, first how to “cut” the S(Q,ω) to retain only the “gain” (positive) part of the S(Q,ω) spectra, then we will explain how to “deconvolve” with the resolution function, which in time domain reduces to a division by the I(Q,t) of the resolution function, and at last to normalize by the I(Q,t=0) value.

    The notion is as follows:

    Nsqt the number of the file(s) having your I(Q,t) data
    Nvan the number of the file(s) having your vanadium data
    Nsqtv the number of the file(s) having your I(Q,t) divided by vanadium
    Nnorm the number of the file(s) having your normalization constant
    Fast Fourier Transforamtion of Data. Only gain side
    mcd To delete the loss part
    3 to select by x value
    0-100 to keep only the positive values. Now we can do the fft...
    tff Fourier Transform in cosinus (we want the real part!)
    t Output coordinate is time
    ps the output unit is generally in the ps scale for TOF measurements
    1e-12 a picosecond is 10-12 s
    S(Q,t) the output coordinate is S(Q,t)
    that is its unit: none
    Deconvolve the instrumental resolution
    glx to put x axis of graphics in a log scale
    p Nsqt You can plot your nice s(q,t). Now we are ready to divide by the resolution
    oy to recalculate y
    / to divide by
    y2 to catch a value per channel
    Nvan the number of the file with your vanadium data.
    Normalize S(Q,t)/S(Q,t=0)
    Nsqt we go to the number of file you have your S(q,t) (already divided by vanadium!)
    oi I will keep a file with only one point per spectrum (the first) which is most similar to S(Q,t=0)
    3 one y value
    1 from the first channel of each spectrum. Now we get a file with a point for every spectrum.
    Nsqtv now we select the file where our data are
    oy we will divide by the points in the file we have just created with one point per spectra
    / divide
    i one point of file Nnorm per spectra
    Nnorm the file with your normalization constants (that we have calculated as the first point of our S(Q,t)

    That’s all folks!

    Fitting a function

    We will now try to fit a function, once a S(Q,ω) is generated. First of all we must create with the command cc the fitting funcion, and afterwards we will fit the function convolving with a vanadium measurement which should have been generated with the same grid (Q,ω) as the data which in turn has to contain a point at zero energy transfer.

    I will suppose that we have in 1 the data, and in 2 the vanadium file.

    1 because we want to fit data in file 1
    cc create the fitting function
    enter or number (65) presing enter you will list the (almost infinite) options. Write the number to select one. as example let’s fit two lorentzians with backropund (number 65)
    1 initial value for a1 intensity of the first Lorentzian
    1 initial value for w1 width of the first Lorentzian
    0 initial value for a2 intensity of the second Lorentzian
    1 initial value for w2 width of the second Lorentzian
    1 initial value for c center for the lorentzian
    3 I will start with only one lorentzian. Therefore I will fix the value for a2=0. Note that to not have bad surprises, w2 is set to some npn-zero value.
    ? if you want to get the options to fix, make the absolute value and so on...
    s a2 is fixed to 0
    enter to go out from the a2 parameter menu
    enter to go out from the parameter fitting menu
    -1 1 I will fit the quasielastic range
    enter
    y I will weigh with the y error
    enter I will NOT weight with log
    y I will convolve with Vanadium
    cf Start the fitting
    enter If you did everything correctly, the option is automatically correct. If not just look at the files where you have your data
    2 Convolve with vanadium, which is in file 2
    enter I will fit ALL the spectra. If a spectra does not contain the value zero, it will simply give an error, and will not fit this spectrum
    numbers, numbers and more numbers because of the fitting process. Normally it is nice to repeat the fitting again. The first number (in brackets) is the reduced chi2. It should be around 1. The other numbers are the fitted parameters.
    cp I will set the intensity of the second lorentzian free. cp is to change the parameters
    3 I will change parameter 3
    s Free at last!!!!
    enter I do not want to change any other parameter
    cf Keep on running... the fitting, and now... extract parameters
    Extracting parameters

    I will try to calculate the diffusion coefficient. I will extract the with of the former fitting, and plot it as a function of Q2, and fit a line...

    3 We should have the results from the former fitting in file 3
    ci I will extract some parameters, exactly
    2,4 the widths of the lorentzians
    meV units of the first width
    meV units of the second width
    done!! as easy as that. Now in files 4,5 you should have w1 and w2
    4,5 I will work with 4 and 5, that is, the extracted widths
    p+enter+enter to have a look... they look nice, then
    ox we will change the x axis
    ^2 square
    6,7 to switch to 6,7 files
    p to plot the beauty of the diffusion coefficient (only if you get straight lines)
    6 let’s fit a line to w1
    cc
    1
    0 zero for origin of the line
    1 for the slope
    5xenter standard values. Sometimes, it happens that the widths do not have errors which will result in “more free parameters than data points”. Come back to this setup with “ca” and disable “weighing with errors”.
    n DANGER!!!!! if you have fitted with convolution you MUST deactivate the convolution. Anyway when the fitting asks you you can also say convolve with file 0, and it will not convolve
    cf

    and finally you will get the value of your diffussion coeffcient extracting the value from the slope in meV·Å2. To get it in 10-4 cm2/s, multiply it by 1.519

    Plotting parameters as a function of a z coordinate, such as temperature

    • After finishing fitting, how can I quickly see the fits together with the data I have already displayed?
      • Type “a”, then the program will “add” the plots for the fittings...
    • I get strange spikes (exactly where the exp. points are) when plotting fitted curves!
      • This is a bug in the “online” convolution with the vanadium measurement. You have to convolve “by hand” using cg, then set the displaystyle to line with eg and add with a.
    • I have done a fitting to several files (i.e. W as a function of Q2), I have extracted the coefficients of the fitting (therefore I have a lot of files with only one value related to the temperature) and now I want to put all them in one file, and plot them as a function of temperature... If the files with one value resulting for the fitting are from number 1 to 10:
    1-10 the files with one extracted value resulting from the fitting
    mfj we will put all the values in one single file. This file will have 10 “spectra” with one only value... and I want to have one spectra with all the values. Therefore
    msx will exchange the values of z (the spectra) and x. BUT sometimes you will get the error message: x ↔ z makes no sense if there are several z’s. Tha is because (of course) because internally the program has got several z’s Then:
    ez we will see if that is true, you should get only one column, if not:
    d to delete one z coordinate
    2 the number 2, for example
    d sometimes you will have also to delete another one
    2 which after deleting the former is now number 2
    enter to go out form the menu
    msx and now you can at last have one nice file with your values as a function of temperature!!!!

    Calculating the diffractogram

    There are more then one ways to calculate the diffractogram. We think that the correct one is the following:

    Integration over Energy

    Probably correct

    Start with d2σ / dΩ dE (choose to generate sig(2th,w) in rtof).

    oi We want to integrate over all energies.
    4
    * We do exactly the same as a Diffractometer and integrate over all energies.
    1

    Now you have something like dσ / dΩ.

    Alternatively

    The following procedure calculates S(2θ). In a second step, it is transformed into S(Q). However, this step is not correct as the detectors (fixed at a certain angle) do not record neutrons with some special Q – Q rather depends on the energy transfer of the sample. Therefore, one would need to transform the spectra first into S(Q,ω) having the problem of a very limited ω-range.

    Start with S(2θ,ω).

    oi We want to integrate over all energies.
    4
    0 1000 Using this, we integrate over all energy-gain-neutrons. The problem with the energy-loss-side is that it depends on the incident wavelength. In other cases, the whole range («*») or the elastic line might be better.
    1

    This is S(2θ) = ∫-∞ dω S(2θ,ω). So far, so good. Now comes the part which is worth discussing,

    Conversion to Q

    Convert x-axis explicitly
    ox We want now to get S(Q), and not S(2θ)
    *
    ec
    0.008726646 to multiply by 0.5*pi/180=0.008726646 (and get theta)
    ox
    sin
    y to take the sine of x
    ox
    *
    ec
    5.02655 if you have lambda=2.5, we must calculate 4*pi/lambda=5.02655
    Alternatively

    Trusting a build-in which sometimes works only the second time called correctly (produces reasonably-looking output the fist time also, though):

    ox
    q Function?
    ec 2nd argument? (this argument is the incident energy)
    2.2725 Its name / value? (81.81/λ2)
    meV Its unit
    Convert y-axis

    The x-axis is now converted to Q, next we have to convert also the y-axis to get a bit closer to the goal that ∫0180 d(2θ) S(2θ) = ∫0Qmax d(Q) S(Q)

    oy Operate on y
    * Multiply
    ec We work only on one file, therefore «ef» would be equally fine
    85.944 45*λ/π (originally: 180/(4*π/λ) )
    0 no error
    85.944 Its name / value
    1 Its unit
    DANGER DANGER DANGER DANGER DANGER DANGER DANGER DANGER DANGER DANGER DANGER DANGER

    In the diffractograms created in this way there are more points than one per q. In order to get a value per q, please, do use mco in order to average points of equal q. Otherwise you can have problems with some instructions (and the graphics without mco are uglier!!!, are more noisiy, of course!)

     

     

     

    CALCULATE S(Q,ω)

    N write the number of file to convert to S(Q,ω)
    _ if you want to know the options. However our is _coq
    _coq
    1 we want a linear grid
    -1.4 the first energy. if the number is too big an error wil occur and frida will tell us the maximum value
    0.01 the step in energy
    100 the last energy you want
    1 equidistant q
    0 the first q
    0.1 Step in q
    10 a big number... to get all the spectra
    30 Number of slices
    0.05 radius of arc... half the value of increment of q 0.1/2=0.05 (high level arithmetics!!!)
    1 minimla number of channels
    TODO: subtract empty can
    Obtain diffusion coefficient

    Please have a look at Fitting a function

    Plotting the two components of a two-Lorentzian fit

    It goes along the lines: export all fit parameters with ci. create two new fit files to the data file. when asked for starting values, say “from file” (check help for how to do that) and use these values. only the ones for the first lorentzian in the first fit file, only the ones for the second in the other. do not fit! simply plot those fit files, convolve with the vanadium.

    To calculate the susceptibility

    Susceptibility is defined as:

    Xq ' ' (w)=S(q,w)/(exp(hw/KBT)-1)-1

    and Xq it is expected to factorize as: Xq(w)=hq*X(w) (see PRE 61 3 2730 (2000), Wuttke et al.), confer to Making a master curve

    _sg to convert input y(x,z) to another representation
    10 option to calculate the susceptibility
    msa To group detectors to have less spectra with better statistics
    z to group by z values, i.e. by detector angle in this case
    10 the tolerance: will group detectors every 10 degrees in 2*theta
    y proceed using channel numbers. this is done because the points in different spectra are not at the same energy. this may cause problems. alternative: put the data on a common grid beforehand using mgr. however, be also aware of mgr, it sometimes does strange things.
    ?? you must write the number of the file where you have the susceptibilty files
    tu convert axes. We use it to “translate” from meV to frequency
    1 more energy = higher frquency
    Hz new unit of x (frida knows it)
    Hz-1 new unit of y

    Now we will substract the empty can

    oy to act on the y axis
    “N1” to act in file number N#. (from now on N wiull be the number of file we are acting on)
    - Funtion - to substract
    y2 An y axis from another file
    N2 To substract the y axis from file number 2 (we are doing, therefore Yn1-Yn2). A lot of numbers will then appear. This is the same issue as above with the channel numbers. if it does, be aware and check the result carefully before submitting to phys rev lett.
    df you can se what you have done (hopefully!)

    To calculate the Density of States (alias DOS)

    SECTION UNDER CONSTRUCTION DO NOT TRUST TOO MUCH WHAT IS WRITTEN!

    In a liquid the density of states is defined as:

    p(w)=2Mh(!/KT) w2 · lim|k|→0 Sinc(q,w)·exp(hw/KT)/q2 (Squires, Pg 102)

    We will therefore, first calculate the s(q,w) in a large range of q (where the elastic line in fact dissapears). Then we will seight spectra by 1/q2, then add the spectra, and then calculate the density of states in the one phonon approximation.

    fl * we load the corrected data s(2theta,w)
    _coq first let’s calculate s(q,w), but fur all q values, i.e. also for long energy transfer regions (for more details see calcuate s(q,w)
    1 linear
    0 initial energy
    0.1 step in energy
    100 max energy (or any other number)
    1 interp in q
    0 q(1)
    0.1 step in q
    10 max q
    30 slices
    0.05 hlaf the q step
    1 min num of channels

    At this point you should have in 2 s(q,w) (or we will suppose it is in file 2)

    2 let’s work in file 2, where s(q,w) is saved
    oy we operate on y (intensity)
    / we want to divide (by q)
    z we ant to divide by z, i.e. by q. You will generate a file 2.
    oy again the same because we want to divide by q2
    / aserejé
    z needs further explanations?. In file 4 we have now s(q,w)/q2 (hopefully!)
    mgi We put all data inthe same grid, otherwise we will run into problems when we add all spectra, because channels are not equal for each spectra!
    1 linear
    0 firs energy
    0.1 steop of energy
    100 last energy. If you had a look, the programs proposes the same numbers as when you calculated s(q,w) (isn’t it intelligent the program?)
    1 linear interpolation. And in file 5 you should now have all the data niceñly in the same grid
    msa we add the spectra
    z let’s group by z value
    100 a hughe number, because we want to add ALL spectra in one single spectra
    1 You must add over full area. In file 6 you should now have your nice s(q,w). Let’s now calcualte g_1(w)
    _sg spetial options
    3 to calculate g_1(w)
    n why not?
    0 why not?
    0 why not?. And we get g_1(w). Now we are going to normalize it. We therefore calculat the integtral
    oi to make the integral
    4 option to integrate
    0 40 the range to integrate (you should have had a look at your data first ;-)
    1 save result as a file, and i nfile 8 you will have the integral result (one point)
    7 finally we will divide by the integral value, so let’s go to 7, where g_1 is calculated
    oy tu divide by the integral
    / divide
    if we will divide by the value in file 8, the integtral
    8 from file 8
    9 here is somethng like the density of states

    The procedure is not fully tested and may have errors!

    Making a master curve

    I have a lot of (nice) spectra, and I would like to make a master curve, that is to say, I would like to divide (multiply) each spectra by a number, to match all the curves in one... I suppose that the spectra are in file 1

    oi to select “integral” options
    9 wit this option you will “rescale” all the curves from a file in one. with option 11 you will match spectra from different files
    n/y do you want to “calculate” the coefficients for the rescaling using the same x-rang for all the curves? let’s asume that you answered y
    * all the range for average
    1 the rescaling coef for curve 1 will be 1
    1 I will save the coeffs in a file... then the program will generate a file (number 2 in this case)with a point per spectra, which is the rescaling factor, and now I want to dicide each spectra by this factor
    1 I will operate on file 1
    oy I will operate on y axis
    / I will divide by the factor in the file
    i The second argument will be one value per spectra taken from another file
    2 from file 2, where I have my rescaling factors

    And now a new file with your rescaled curves is created.

    Redistributing data or the joy of mgr

    Once we have some file we would like to redistrivute the data and put them in a regular grid (for example from (q,w) or w in a logarithmic scala...), how to do it? As an example we will “put the data” in a x logarithmic grid.

    mgr to redistribute the data
    3 because we want to redistribute in a logarithmic scala (1 for a linear one!!!), if you want to make linear for a determined range, and log for another range use option 2
    1e10 for the first value (it is so big because in our case was a frequency in Hz...)
    1e13 for the last value
    10 I want 10 point per decade. In the case of a linear grid the increment of x axis will be asked
    3 What happens if there are not enough experimental points to “fill” the grid. with option 3 the program will fill the whole grid, also if there are no experimental points, repeating the value of the closer point. Option 2 will not fill the whole grid with points... just test it!!!
    here you have it!!!

     

     

     

    Generating a function

    The thing is simple: I simply want to generate a function (in my case an exponential y=exp(-x*w)) to compare with data, therfore I want to generate it in the sam x spacing as the data. In addition I want to take the values from a file with the w data (froma a fitting for example). I do NOT know if that is the easiest way to do it, but it seems to work ;-) If I am working at file 1:

    oy I will act in the y axis
    50 I will copy the values of x in the y axis (I am now choosing to get the values from “somewhere”)
    x I copy the x data (this “somewhere” is the x data). And now I have a new file with y=x values
    Nx=y I will operate in the redcently new created Nx=y file with y=x, therefore all what I do with y, I do it with x
    oy * multiply
    ec -1 we multiply by -1 to get -x
    i second argument is a point per file (in our case w)
    Npar from the file with parameters, and we get -x*w
    oy modifiy y
    3 to selsect the function exponential

    and finally we get exp (-x*w)