# Background charge in PSI4

Is it possible to run calculations (energy, optimization, etc) with background charge? I have tried to do this with External potentials with HF, but it cannot reproduce the result yielded with Gaussian charge function.

I don’t recall which, but the external potentials process you linked to had been checked against another qc program. You’ll have to supply the Gaussian result if you want us to try to match it.

Hi Lori,

Thanks for replying. I will give my G09 and Psi4 input files below.

PSI4 input:

``````molecule water {
0 1
O  -0.778803000000  0.000000000000  1.132683000000
H  -0.666682000000  0.764099000000  1.706291000000
H  -0.666682000000  -0.764099000000  1.706290000000
no_reorient
no_com
}
``````
``````Chrgfield = QMMM()
psi4.set_global_option_python('EXTERN',Chrgfield.extern)
``````
``````set {
scf_type direct
reference rhf
d_convergence 12
basis cc-pvdz
}
``````
``````energy('scf')
``````

Gaussian Input:

``````#p HF/cc-pvdz  charge
``````
``````Comment
``````
``````0 1
O  -0.778803000000  0.000000000000  1.132683000000
H  -0.666682000000  0.764099000000  1.706291000000
H  -0.666682000000  -0.764099000000  1.706290000000
``````
``````  1.649232019048  0.0  -2.356023604706 -0.834
0.544757019107  0.0  -3.799961446760  0.417
0.544757019107  0.0  -0.912085762652  0.417
``````

Results:
PSI4:

@RHF Final Energy: -76.03498602087275

=> Energetics <=

``````Nuclear Repulsion Energy =              9.4264780387548672
One-Electron Energy =                -123.3805014956179633
Two-Electron Energy =                  37.9190374359903402
DFT Exchange-Correlation Energy =       0.0000000000000000
Empirical Dispersion Energy =           0.0000000000000000
PCM Polarization Energy =               0.0000000000000000
EFP Energy =                            0.0000000000000000
Total Energy =                        -76.0349860208727648
``````

G09:

SCF Done: E(RHF) = -76.9673137369 A.U. after 11 cycles
Convg = 0.2117D-08 -V/T = 2.0117
KE= 7.607945147638D+01 PE=-2.003825938821D+02 EE= 3.807995635848D+01

I have tried running calculations without background charges in both programs, and the results agree with each other.

I wonder if it is the way I call calculations with background charge in PSI 4 is wrong.

Thanks a lot.

Chester

I understand where wnet wrong: the computation results with Gaussian include the self-energy of background charges, while PSI4 does not consider that; also a nosymm keyword should be added in Gaussian input file to avoid further reorientation.