RESP charges

Ainosya · 2026-07-16 18:35:57

Hello,

I am trying to compute RESP charges using ORCA, ORCA_MOLDEN + Multiwfn, and Gaussian09 (G09).

Although ORCA and G09 give very similar results, ORCA does not allow me to impose constraints on equivalent charges. For example, the three O atoms in my O–O–O group do not have the same charge compared to the G09 results.

However, when using Multiwfn (and applying charge constraints), I obtain significantly different charges. I initially thought this difference was caused by the constraints I applied, but I obtained similar charges even without using any constraints in Multiwfn.

I understand that the ESP fitting procedure can differ between ORCA, G09, and Multiwfn, and therefore small differences in the fitted charges are expected. However, while ORCA and G09 give very close results, the charges obtained from ORCA_MOLDEN + Multiwfn with and without constraints are significantly different in a way that seems too large to be explained by small fitting differences.

I also tried increasing the number of grid points per Ångström in the Merz–Kollman (MK) fitting grid in Multiwfn, but the resulting charges remain essentially unchanged.

Is there something I might have missed in the setup or procedure?

# ORCA G09 Multiwfn+Constraint Multiwfn - NO constraint
C -0.10523418 -0.057192 -0.0302549631 -0.0746656346
C 0.70500075 0.699589 0.6288807467 0.6555595774
C 0.10433265 0.083272 0.1741338975 0.1577934901
C 0.44319808 0.397013 0.4040619582 0.4102463676
C 0.09260651 0.143337 0.1492221951 0.1392991455
O -0.63725710 -0.614485 -0.6895841082 -0.6881095374
P 1.48406838 1.333977 1.7508945226 1.7589400745
O -1.00352353 -0.943652 -1.0690527494 -1.0869672209
O -0.97997653 -0.943652 -1.0690527494 -1.0629012194
O -0.98469247 -0.943652 -1.0690527494 -1.0728677364
H 0.03449197 0.018788 0.0254453388 0.0255041882
H 0.02538917 0.018788 0.0254453388 0.0255041882
O -0.88667523 -0.843311 -0.8813434864 -0.8940507921
H 0.56367069 0.518545 0.5691170437 0.5918946082
H -0.02545746 -0.017223 -0.0060522160 -0.0072544242
O -0.76411480 -0.748446 -0.8027866739 -0.7921751928
H 0.44587575 0.440245 0.4486240861 0.4420728698
O -0.67290047 -0.667816 -0.6580181465 -0.6622031133
O -0.50653098 -0.492425 -0.4962606888 -0.4796046960
P 1.37887179 1.252465 1.5714956904 1.5732465978
O -0.95602235 -0.921277 -1.0196082991 -1.0208165689
O -0.95907728 -0.921277 -1.0196082991 -1.0250970345
O -0.95528613 -0.921277 -1.0196082991 -1.0146631266
H 0.06662773 0.056874 0.0528663938 0.0607384487
H 0.07925746 0.056874 0.0528663938 0.0607384487
H 0.01335758 0.015917 -0.0227701773 -0.0201617078

Thanks for your insight

sobereva · 2026-07-16 18:52:20

ORCA and Gaussian cannot produce RESP charges in common sense. If you just need RESP charges (usually for MD simulation), just follow the standard steps in Multiwfn to compute them. There has been much more than one thousand of papers using Multiwfn to compute RESP charges.

The difference without constraints may come from multiple reasons, such as difference in grid distribution, fitting scheme (one or two-stages), etc.

RESP fitting is involved. In constrast, you can find MK charges computed by Gaussian and Multiwfn are quite close to each other.

Ainosya · 2026-07-16 19:28:40

Hello,

I am not sure what you mean by G09 being unable to compute RESP charges. Both the ORCA and Gaussian09 manuals provide the necessary input to compute them.

For Gaussian09, the input is as follows:

```
#P HF/6-31G* Pop=MK IOp(6/33=2) IOp(6/41=15) IOp(6/42=15) IOp(6/50=1)
# Test Units(Ang,Deg)
```

I am also using the MK method with Multiwfn, yet there is still a large discrepancy compared to ORCA, which uses COSMO I believe

Ainosya · 2026-07-16 19:30:29

The issue is not that Multiwfn with and without constraints gives different results. This seems reasonable and consistent.

The main question is why ORCA and G09 give similar results despite using slightly different fitting procedures, whereas Multiwfn is completely different compared to both.

sobereva · 2026-07-17 04:59:45

Ainosya wrote:

Hello,
I am not sure what you mean by G09 being unable to compute RESP charges. Both the ORCA and Gaussian09 manuals provide the necessary input to compute them.
For Gaussian09, the input is as follows:
```
#P HF/6-31G* Pop=MK IOp(6/33=2) IOp(6/41=15) IOp(6/42=15) IOp(6/50=1)
# Test Units(Ang,Deg)
```
I am also using the MK method with Multiwfn, yet there is still a large discrepancy compared to ORCA, which uses COSMO I believe

Please carefully look at Kollman's original paper of RESP charge (J. Phys. Chem., 97, 10269 (1993)), and relevant part of Multiwfn manual (Section 3.9.16). RESP charge is not a variant of ESP fitting charge just with additional constraints, but it defined specific fitting scheme. The so-called RESP charge in ORCA and Gaussian only enable users to manually set simple constraints, while it doesn't directly follow the full procedure of RESP charge fitting in Kollman's paper, so the results are not comparable.

Even for MK charges, which is one of the simplest ESP fitting charge, the results of Multiwfn and ORCA/Gaussian can also be detectably different even based one exactly the same wavefunction, because the spatial distribution of fitting points are not completely identical in these programs. ESP fitting charge is known to be sensitive to distribution and density of fitting points. In Multiwfn, the default density of fitting points is fine enough and can lead to basically converged result, and the point distribution strictly follows the original MK definition.

Multiwfn forum

#1 2026-07-16 18:35:57

RESP charges

#2 2026-07-16 18:52:20

Re: RESP charges

#3 2026-07-16 19:28:40

Re: RESP charges

#4 2026-07-16 19:30:29

Re: RESP charges

#5 2026-07-17 04:59:45

Re: RESP charges

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