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From: Joel Subach (mjsubach_at_alumni.ncsu.edu)
Date: Sat May 11 2024 - 10:55:20 CDT

..answer to above: yes RI and RIJCOSX exhibit errors magnitudes smaller
than the intrinsic error in MP2 or the basis set or the force field
simulation itself.

On Sat, Apr 27, 2024 at 9:04 AM Joel Subach <mjsubach_at_alumni.ncsu.edu>
wrote:

> 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.´
>
> 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: 🚀]
>