9.4 Application of Pressure

9.4.2 eXtended Hydrostatic Compression Force Field (X-HCFF)

(May 16, 2021)

The eXtended Hydrostatic Compression Force Field (X-HCFF) approach was introduced by Stauch to solve the problems associated with HCFF. 1037 Stauch T.
J. Chem. Phys.
(2020), 153, pp. 134503.
In X-HCFF, mechanical forces are used to compress the molecule as well, but, in contrast to HCFF, these forces are strictly perpendicular to the tessellated molecular surface, thus simulating truly hydrostatic conditions. As a result, chemically feasible geometries are retained even at high pressures. In addition, the user is able to input the precise pressure that is applied to the molecule during the simulation. It was suggested to use the unscaled atomic van der Waals radii in the tessellation routine. 1037 Stauch T.
J. Chem. Phys.
(2020), 153, pp. 134503.
X-HCFF works with any electronic structure method for which a nuclear gradient is available.

As in HCFF, the application of pressure to atoms cannot be modeled realistically with X-HCFF, and the observed pressure-induced increase in electronic energy is typically too low.

Example 9.11  Geometry optimization of the CO2 dimer under a pressure of 100 GPa using the X-HCFF model

   0 1
   O    2.6192991230   -0.0571311942    0.0000000000
   C    1.6782610262    0.6502025480    0.0000000000
   O    0.7413912820    1.3674070371    0.0000000000
   C   -1.6782610262   -0.6502025480    0.0000000000
   O   -2.6192991230    0.0571311942    0.0000000000
   O   -0.7413912820   -1.3674070371    0.0000000000

   JOBTYPE            opt
   METHOD             pbe
   BASIS              cc-pvdz
   DISTORT            true

   model              xhcff
   pressure           100000
   scaling            1.0
   npoints_heavy      302
   npoints_hydrogen   302

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