Generate RESP charges with Gaussian

In case one uses molecules unrecognized by Amber, a restrained electrostatic potential (RESP) charges calculation might be needed. Here is how to do it with the Gaussian package.

First optimize the structure of the desired molecules. Here we use n-hexane and optimize it at the DFT/B3LYP/6-31G(d,p) level using 16 cores and 4000MB of memory per core.

geo_opt.com:

%NProcShared=16
%chk=./geo_opt.chk
%mem=4000MB
#p opt b3lyp/6-31G(d,p) nosymm iop(6/7=3) gfinput
n-hexane geometry optimization

0 1
C 0.7746 -0.0257 0.0000
C -0.7746 0.0257 0.0000
C 1.3570 -1.4627 0.0000
C -1.3570 1.4627 0.0000
C 2.9005 -1.5027 -0.0000
C -2.9005 1.5027 -0.0000
H 1.1449 0.5037 0.8825
H 1.1449 0.5037 -0.8825
H -1.1449 -0.5037 -0.8825
H -1.1449 -0.5037 0.8825
H 0.9985 -2.0010 0.8818
H 0.9985 -2.0010 -0.8818
H -0.9985 2.0010 0.8818
H -0.9985 2.0010 -0.8818
H 3.2506 -2.5369 0.0000
H 3.2996 -1.0073 -0.8871
H 3.2996 -1.0073 0.8871
H -3.2506 2.5369 0.0000
H -3.2996 1.0073 -0.8871
H -3.2996 1.0073 0.8871

Run Gaussian as follows:

module load g09
nohup g09 < geo_opt.com > geo_opt.log &

Once the geometry has been optimized we can calculate the RESP charges with the following input.

electrostatic_grid.com:

%NProcShared=16
%chk=./electrostatic_grid.chk
%mem=4000MB
#p b3lyp/6-31G(d,p) nosymm iop(6/33=2) pop(chelpg,regular)

n-hexane electrostatic grid calculation

0 1
C   0.766190   -0.024729    0.000000
C  -0.766190    0.024729    0.000000
C   1.335815   -1.448705    0.000000
C  -1.335815    1.448705    0.000000
C   2.866858   -1.488377    0.000000
C  -2.866858    1.488377    0.000000
H   1.147135    0.517440    0.877538
H   1.147135    0.517440   -0.877538
H  -1.147135   -0.517440   -0.877538
H  -1.147135   -0.517440    0.877538
H   0.955507   -1.990023    0.877033
H   0.955507   -1.990023   -0.877033
H  -0.955507    1.990023    0.877033
H  -0.955507    1.990023   -0.877033
H   3.241978   -2.516956    0.000000
H   3.275854   -0.985176   -0.883579
H   3.275854   -0.985176    0.883579
H  -3.241978    2.516956    0.000000
H  -3.275854    0.985176   -0.883579
H  -3.275854    0.985176    0.883579

And the following command:

nohup g09 < electrostatic_grid.com > electrostatic_grid.log &

Where we have copied the geometry from the previous calculation. The electrostatic_grid.log output file can then be used, e.g. to generate a .mol2 file for leap and Amber.