# How to calculate sigma-profile using COSMO/RS and Gaussian 16?

Gaussian 16 claims to have an external tool to calculate sigma-profile COSMO/RS (Klamt) [1]. In the support website, there is a SCRF flag for COSMORS [2].

How do I calculate sigma-profile using Gaussian 16 and COSMO/RS? Are there any examples available?

Gaussian itself doesn't have any way of computing sigma-profiles on its own. As you noticed, it has the scrf=COSMORS keyword, but this just produces a file that is used by a dedicated program (e.g. COSMO-RS, COSMOtherm).

On the Gaussian side, it really is as straightforward as using that keyword. Here is a minimal example:

#p B88-VWN-P86/TZVP scrf=cosmors

peroxide

0 1
O  -0.22216884 -0.6214829  -0.08709297
H   0.09625091 -1.07514452  0.69674375
O   0.22216884  0.6214829  -0.08709297
H  -0.09625091  1.07514452  0.69674375

h2o2.cosmo


This produces the necessary .cosmo file. Note the choice of functional and basis set is important, as COSMOtherm only ships parameter sets for certain combinations. In this case, you need to be sure the geometry is converged at this level of theory to use the BP_TZVP_C30_1701.ctd parameter file. If you can't converge it at that level of theory, you can use the SVP basis set and a different parameter file (BP_SVP_AM1_C30_1701.ctd).

I don't have access to either of the dedicated COSMO programs, but at least the input for COSMOtherm seems pretty straightforward. In an input file (name.inp), you would at minimum write:

ctd=name.ctd
#Identifier

f=name.cosmo


This would be run using cosmotherm name.inp from the commandline.