The Hydrostatic Compression Force Field (HCFF) model was introduced by Stauch, Chakraborty and Head-Gordon.1067 In HCFF, mechanical forces that point towards the non-mass-weighted molecular centroid are used to compress a molecule. Care must be exercised when modeling extended molecules due to the tendency of HCFF to generate spherical geometries under very high pressure.1070 Also, the pressure input by the user is only a guess for the pressure that is applied to the molecule. The latter is calculated a posteriori based on the generated geometry and the molecular surface and is output as HCFF Macroscopic Pressure. Typically, the applied pressure is lower than the input pressure. It should be noted that the dependence on the nuclear gradient precludes the application of pressure to single atoms in HCFF. Moreover, the increase in electronic energy when compressing a molecule is typically underestimated by HCFF, since the pressure acts only on the nuclei, whereas the compression of electron density is not modeled directly. HCFF works with any electronic structure method for which a nuclear gradient is available.
Example 9.10 Geometry optimization of diborane under pressure using the HCFF model with an input pressure of 3808 MPa
$molecule 0 1 B 0.0000000000 0.0000000000 0.8917854534 B 0.0000000000 0.0000000000 -0.8917854534 H -0.5244343500 0.9105724300 1.4720415209 H 0.5244343500 -0.9105724300 1.4720415209 H -0.5244343500 0.9105724300 -1.4720415209 H 0.5244343500 -0.9105724300 -1.4720415209 H 0.8561835151 0.4929549655 0.0000000000 H -0.8561835151 -0.4929549655 0.0000000000 $end $rem JOBTYPE opt METHOD m06-2x BASIS 6-311++G(d,p) DISTORT true $end $distort model hcff pressure 3808 scaling 1.0 npoints_heavy 590 npoints_hydrogen 590 $end