# GAPW and GTH

Harald Forbert harald.... at theochem.ruhr-uni-bochum.de
Tue Jul 3 19:21:46 CEST 2007

```Sorry, it took me a while to test things, but EPSFIT at 10^-2 seems to
work really well. Thanks a lot. :)

Though while testing I noticed something 'odd':
If I use H2O instead of HCl (and essentially the same input as above),
the results also look good, the grid effect is quite a bit smaller
than the
default EPS_FIT, MD seems stable, but if I just do a energy_force
calculation of the following configuration:

O 0.0 0.0 0.0
H 0.0 0.59124 -0.77291
H 0.0 0.59124 0.77291

I get a strange force on the oxygen in z direction:

ENERGY| Total FORCE_EVAL ( QS ) energy (a.u.):
-17.211597494294843
FORCES| (a.u.)
O         0.0000000127       -0.0024393452        0.0001621868
H        -0.0000000043        0.0012914592        0.0010373482
H         0.0000000041        0.0011673908       -0.0011995375

Now, the force isn't that large, but if I simply switch axis (from z
to x) to:

O 0.0 0.0 0.0
H -0.77291 0.59124 0.0
H 0.77291 0.59124  0.0

I get basically the same energy, and the force on oxygen looks much
better as
one would expect from symmetry:

ENERGY| Total FORCE_EVAL ( QS ) energy (a.u.):
-17.211597494294828
FORCES| (a.u.)
O        -0.0000000021       -0.0024393452        0.0000000127
H         0.0011184426        0.0012294258       -0.0000000043
H        -0.0011184430        0.0012294243        0.0000000041

How come the different axis give such different results? Is it really
just the ordering
of the loops over axis in the code? I wouldn't have thought the impact
quite that
large from -2.1e9 to 1.6e-4....
Or is this an unlucky extreme example (due to symmetry)?

BTW the only force term that is really different in both calculations
is the vhxc_atom part and
for default EPS_FIT the difference is far less pronounced.

-H

```