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# 9.4.1 Introduction

(February 4, 2022)

Q-Chem features a number of methods to apply pressure to a chemical system during a geometry optimization or an AIMD simulation.1070 The following methods are implemented:

 Hydrostatic Compression Force Field (HCFF)1067 Section 9.4.2 eXtended Hydrostatic Compression Force Field (X-HCFF)1069 Section 9.4.3 Gaussians On Surface Tesserae Simulate HYdrostatic Pressure (GOSTSHYP)997 Section 9.4.4

To invoke one of these methods, set DISTORT = TRUE in the $rem section. By setting the$rem variable scf_final_print = 1, the energy contribution due to pressure is printed in the output.

DISTORT

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 Field1067 XHCFF eXtended Hydrostatic Compression Force Field1069 GOSTSHYP Gaussians On Surface Tesserae Simulate HYdrostatic Pressure997 EFEI External Force is Explicitly Included (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.997 A value of 1.0 was suggested to be used in the X-HCFF model.1069

Gostshyp_Mem
Specifies the amount of memory, the gostshyp model is allowed to allocate in MB.
INPUT SECTION: $distort TYPE: INTEGER DEFAULT: $66~{}\%$ of MEM_TOTAL OPTIONS: User defined RECOMMENDATION: Set larger than expected gostshyp memory usage. As a rule of thumb gostshyp will need $5\cdot N_{Basis}^{2}\cdot N_{tess}$ Bytes memory. screener Enables/disables Integral screening for gostshyp calculations. INPUT SECTION:$distort
TYPE:
BOOL
DEFAULT:
true
OPTIONS:
true Enable integral screening for gostshyp false Disable integral screening for gostshyp
RECOMMENDATION:
Use default. Disabling integral screening will lead to much higher memory usage and severe performance drops.