VMD-L Mailing List

From: Joel Subach (mjsubach_at_alumni.ncsu.edu)
Date: Sat Apr 27 2024 - 02:03:24 CDT

Hello VMD Forum,

I am gently optimizing parameters via ffTK where I am executing ORCA to
generate output.out files which are subsequently input back into ffTK
towards this newly generated parametrized species. Via ORCA I instead
executed the below method/basis set successfully to speed up calculations:

*! RI-MP2 6-31G* TightSCF opt autoaux rijcosx*

The default ffTK Method/Base Set is: ! MP2 6-31G* TightSCF opt

The ffTK literature recommends an HF/6-31G(d) level of theory to maintain
consistency with the CHARMM force field, below are two excerpts towards
this quoted:

`These two parameters are optimized quantum mechanically, with all other
degrees of freedom constrained at the HF/6-31G(d) level of theory to
maintain consistency with the CHARMM force field.

`The charges in the additive CHARMM force field are based on the
reproduction of minimum interaction energies and distances between the
compound of interest and water using HF/6-31G*. This is why that basis set
is being used.ยด

I am assuming that the ffTK default ! MP2 6-31G* TightSCF opt is
satisfactory towards the above but according to the above would the RI in
bold above as well be consistent with HF/6-31G(d) level of theory? If yes
then I will leave my results asis to manage time, this is beyond the scope
of my expertise.

Thanks if you know:),
Joel[image: ๐Ÿš€]