[molpro-user] Lattice point of charges in Molpro vs Orca

[Susi Lehtola]

On Tue, 8 Apr 2014 09:36:09 +0300
Susi Lehtola <susi.lehtola at alumni.helsinki.fi> wrote:
> On Mon, 7 Apr 2014 21:20:22 +0200
> Quan Phung <manhquank9 at gmail.com> wrote:
> > Dear Molpro users,
> > 
> > I'm trying to calculate Fe atom with 4 surrounding point charges.
> > The nuclear repulsion energy by Molpro is  -27.56070553 Ha.
> > However, both Orca and Molcas give -26.54614640 Ha.
> > Did I do something wrong?
> 
> First, you seem to be calculating Fe^{2+}, not Fe. Second, the nuclear
> repulsion energy should be positive. Anyway...

The numbers do seem really weird. First, the nuclear repulsion energy
doesn't depend on the electronic structure. Second, electronic
contributions for iron would be much larger than a few dozen Hartree.
But the number is really big to be purely Coulombic.

Computing the numbers by hand I get

- repulsion of the point charges: 1.0145 Ha
- attraction of point charges and nucleus: -1.060 Ha
total: -0.0455 Ha

If you compute the difference between the Molpro and Orca numbers, you
get the difference of 1.0145591 Ha, which you see is the repulsion of
the point charges.

I'm guessing Molpro doesn't include the static repulsion of the point
charges in the energy it computes.
-- 
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Mr. Susi Lehtola, PhD             Research Associate
susi.lehtola at alumni.helsinki.fi   Department of Applied Physics
http://www.helsinki.fi/~jzlehtol  Aalto University
                                  Finland
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Susi Lehtola, FT                  Tutkijatohtori
susi.lehtola at alumni.helsinki.fi   Fysiikan laitos
http://www.helsinki.fi/~jzlehtol  Aalto-yliopisto
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