# 9.4 Application of Pressure

(May 16, 2021)

Q-Chem features a number of methods to apply pressure to a chemical system during a geometry optimization or an AIMD simulation. The following methods are implemented:

 Hydrostatic Compression Force Field (HCFF) 1035 Stauch T., Chakraborty R., Head-Gordon M. ChemPhysChem (2019), 20, pp. 2742. Section 9.4.1 eXtended Hydrostatic Compression Force Field (X-HCFF) 1037 Stauch T. J. Chem. Phys. (2020), 153, pp. 134503. Section 9.4.2 Gaussians On Surface Tesserae Simulate HYdrostatic Pressure (GOSTSHYP) 965 Scheurer M. et al. J. Chem. Theory Comput. (2021), 17, pp. 583. Section 9.4.3

To invoke one of these methods, the $rem variable DISTORT needs to be set to true. DISTORT Specifies whether to apply pressure or external force to a chemical system TYPE: LOGICAL DEFAULT: False OPTIONS: False Do not use pressure or force True Use pressure or force RECOMMENDATION: Set to true to apply pressure or force. The parameters of the jobs are set via options specified in the$distort input section. The format of the $distort section is analogous to the$rem section:

$distort <Keyword> <parameter/option>$end


Note:  The following job control variables belong only in the $distort section. Do not place them in the$rem section.

Model
Specifies which model is used to distort the molecule.
INPUT SECTION: $distort TYPE: STRING DEFAULT: None OPTIONS: HCFF Hydrostatic Compression Force Field XHCFF eXtended Hydrostatic Compression Force Field GOSTSHYP Gaussians On Surface Tesserae Simulate HYdrostatic Pressure EFEI External Force is Explicitly Included (cf. Section 9.5) RECOMMENDATION: Please refer to the following subsections for recommendations on which model to use. Pressure Specifies the pressure (in MPa) used to compress the molecule. INPUT SECTION:$distort
TYPE:
DOUBLE
DEFAULT:
None
OPTIONS:
User defined
RECOMMENDATION:
None

NPoints_Heavy
Specifies the number of tessellation points per non-hydrogen atom.
INPUT SECTION: $distort TYPE: INTEGER DEFAULT: 110 OPTIONS: User defined RECOMMENDATION: Use the default. NPoints_Hydrogen Specifies the number of tessellation points per hydrogen atom. INPUT SECTION:$distort
TYPE:
INTEGER
DEFAULT:
110
OPTIONS:
User defined
RECOMMENDATION:
Use the default.

Scaling
Specifies the scaling factor of the atomic van der Waals radii used in the tessellation of the molecular surface, which is used in the pressure models.
INPUT SECTION: \$distort
TYPE:
DOUBLE
DEFAULT:
1.2
OPTIONS:
User defined
RECOMMENDATION:
Increase when modeling a chemical complex to make sure that the complex is placed inside a single cavity. A value of 1.0 was suggested to be used in the X-HCFF model.