Pressure in bulk system depends on 1-4 neighbour electrostatics ?

[Lukasz Walewski]

Hi,

Recently I was looking at different organic solvents with MM force
field using Fist. I noticed that the internal pressure in the bulk
system (cubic pbc) highly depends on whether I include the
electrostatic interactions between 1-4 neighbours or not. The force
field I used requires that the 1-4 electrostatics is fully included,
i.e. EI_SCALE14 should be set to 1.0. With such a setting and for a
pre-equilibrated system of 360 dmso molecules in a ~35A box at 300K I
get:

  Total Energy               =       -16.1592140101
  Internal Pressure [bar]    =      4744.0521754103

The default in CP2K is however not to include the 1-4 electrostatics
at all (EI_SCALE14 defaults to 0.0) and with this setting and the very
same system/configuration/other options/etc., I get:

  Total Energy               =        -0.2816880077
  Internal Pressure [bar]    =     -2108.3622331873

The difference in energy is of course expected, but common sense
dictates that intra-molecular interactions should not affect the
pressure that much, since it is the property that stems mostly from
inter-particle interactions. At least I wouldn't expect the impact to
be so large since the Coulomb attraction between H and O atoms within
the same dmso molecule modifies it's geometry only slightly and
doesn't affect the inter-molecular interactions at all.

I would be glad to hear you comments on that.

Best regards,
Lukasz