TeraChem User Forum

Please login or register.

Login with username, password and session length
Advanced search  

News:

Kepler Support out in v1.50K!

Author Topic: interpretation of gradient values  (Read 926 times)

Bokhimi

  • Newbie
  • *
  • Karma: +0/-0
  • Posts: 37
interpretation of gradient values
« on: October 24, 2016, 09:35:12 PM »

Because I have troubles using the gradient values given by TeraChem for my system. I compared the gradient values obtained using TeraChem with the corresponding ones obtained with Gaussian (Results are attached). For the comparison I obtained these values for a water cluster made of 6 water molecules.

Why I obtained so different results?

Bokhimi
Logged

ufimtsev

  • Administrator
  • Sr. Member
  • *****
  • Karma: +1/-0
  • Posts: 279
Re: interpretation of gradient values
« Reply #1 on: October 29, 2016, 11:53:17 AM »

Please check your start file, something may be wrong with the method or basis set. Here is a result for

coordinates   water.xyz
charge        0
method        rb3lyp
run           gradient
basis         6-31g*
end


FINAL ENERGY: -458.5384933282 a.u.
CENTER OF MASS: {0.000077, 0.000006, 0.000004} ANGS
DIPOLE MOMENT: {-0.003041, 0.000195, -0.002096} (|D| = 0.003698) DEBYE

Running Mulliken population analysis...

GPU Memory: 9411 Mb
Writing out molden info

Gradient units are Hartree/Bohr
---------------------------------------------------
        dE/dX            dE/dY            dE/dZ
    0.0023710677     0.0051839449     0.0054077283
    0.0074956937    -0.0105356040     0.0035103656
    0.0077306173     0.0004218266    -0.0108471130
   -0.0064421340     0.0004360239     0.0042808221
    0.0083020774     0.0071885933     0.0077152096
   -0.0043217808     0.0045016851    -0.0045524299
   -0.0077132761    -0.0004261841     0.0108439615
    0.0043001102    -0.0045131765     0.0045531296
   -0.0083017346    -0.0071968118    -0.0077279848
    0.0064403922    -0.0004466109    -0.0042795274
   -0.0074943199     0.0105399984    -0.0035002380
   -0.0023718638    -0.0051780596    -0.0054114713
   -0.0056043331     0.0043749254    -0.0029214078
   -0.0032229892    -0.0034493254    -0.0060460568
    0.0052150610    -0.0016924325    -0.0053162523
   -0.0051983044     0.0017065812     0.0053155498
    0.0032208366     0.0034650309     0.0060489377
    0.0055948798    -0.0043804047     0.0029267768
---------------------------------------------------
Logged

Bokhimi

  • Newbie
  • *
  • Karma: +0/-0
  • Posts: 37
Re: interpretation of gradient values
« Reply #2 on: November 01, 2016, 02:55:52 AM »

Thanks Ivan

In the comparison, for TeraChem I used the basis 6-31g and not 6-31g*.

Doing this change in the basis the comparison gave the same gradient values.
Logged

ufimtsev

  • Administrator
  • Sr. Member
  • *****
  • Karma: +1/-0
  • Posts: 279
Re: interpretation of gradient values
« Reply #3 on: November 02, 2016, 11:51:03 AM »

Comparing gradients may be tricky sometimes if you want many digits after the decimal to be the same. First thing -- the SCF convergence threshold should be as tight as possible, since the energy is quadratic in delta C and gradients are linear. In DFT, you really want to run the calculations on exactly same grids. This is often impossible, because different packages use different grids, and it's recommended to use ultra fine grids to minimize numerical noise. I heard that some programs (not TeraChem, of course) don't compute the grid derivatives contribution to the gradient, i.e. their gradients are just incorrect, and you don't want to use them in your tests.
-Ivan
Logged