Molcas Forum

Jtam

Member

Registered: 2026-06-25

Posts: 2

SLAPAF ignoring EXTF gradients during constant force optimization

Hello all,

I am simulating the mechanochemical ring-opening of gem-difluorocyclopropane using OpenMolcas. To simulate the mechanical stress, I apply a constant pulling force (3√2 nN ≈ 4.24 nN) between specific attachment points (H atoms 4 and 9) using the LINEAR within the EXTF module. I'm trying to find to find the force-modified minimum.

I am having trouble getting SLAPAF to properly account for the external force during the geometry optimization. A ~4.24 nN pulling force applied to these two H atoms should be more than sufficient to stretch the molecule and break the C1-C6 bond. However, the optimization steps and the final optimized geometry show absolutely no sign of this happening as the bond does not stretch or break.

It seems that the gradients calculated by the EXTF module are either not being passed to SLAPAF correctly, or SLAPAF is simply ignoring them, meaning the optimizer does not update the geometry according to the applied external pulling force. I have already tried adding the Cartesian keyword to the &SLAPAF module to force optimization in Cartesian coordinates, but this did not resolve the issue.

Is there a specific keyword, loop structure, or workaround required to run SLAPAF for a constant external force optimization?

Input File:

*  CASPT2(2,2)/6-31G* (trans, 3*sqrt(2) nN)

>>> EXPORT MOLCAS_MAXITER = 200
>>> EXPORT MOLCAS_REDUCE_PRT = NO

&GATEWAY
  Coord
     9
 gem-difluorocyclopropane     
 C    -1.90201016    -0.46176426    -0.13357965
 C    -0.60372247     0.23262042    -0.10022135
 F    -0.31171934     1.04814040     0.94636422
 H    -2.56043431    -0.33909258     0.71879870
 H    -2.38737757    -0.57546339    -1.09618575
 C    -0.57518115    -1.22711815     0.09260645
 F    -0.10264035     0.76256834    -1.24641160
 H    -0.37609013    -1.59908634     1.09116641
 H    -0.20303316    -1.83545734    -0.72381801
  Basis = 6-31G*
  Group = C1
  RICD

>>> DOWHILE
&SEWARD
&RASSCF
  Spin     = 1
  Charge   = 0
  NActEl   = 2 0 0
  RAS2     = 2
  Inactive = 19
  CIRoot   = 1 1 1
&CASPT2
  IPEA = 0.0
  GRDT
&ALASKA
&EXTF
  LINEAR
  4 9
  MODULE
  -4.24264
&SLAPAF
>>> ENDDO

Thank you!

Last edited by Jtam (2026-06-25 15:19:16)

Ignacio

Administrator

From: Uppsala

Registered: 2015-11-03

Posts: 1,204

Re: SLAPAF ignoring EXTF gradients during constant force optimization

Thanks for the notice. I think this is an oversight. Try adding the following line to the data/extf.prgm file (once compiled):

 (file) GRADS "$WorkDir/$Project."Grads rw

Jtam

Member

Registered: 2026-06-25

Posts: 2

Re: SLAPAF ignoring EXTF gradients during constant force optimization

Thank you, Ignacio, for the fix and the quick reply. It's working now!