The eXtended Hydrostatic Compression Force Field (X-HCFF) approach was introduced by Stauch to solve the problems associated with HCFF.1069 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.1069 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 CO dimer under a pressure of 100 GPa using the X-HCFF model
$molecule 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 $end $rem JOBTYPE opt METHOD pbe BASIS cc-pvdz DISTORT true $end $distort model xhcff pressure 100000 scaling 1.0 npoints_heavy 302 npoints_hydrogen 302 $end