XC_DEFINITION (.param)

Keyword type

Block

Description

This block allows you to construct a bespoke exchange-correlation potential by mixing contributions from various local and nonlocal functional forms.

The data block has the following format, where the number of functionals is limited only by the number of implemented functional forms and by various constraints described below:

%BLOCK xc_definition
<xc-functional> weight
NLXC_SCREENING_LENGTH  length [optional]
NLXC_SCREENING_FUNCTION function [optional]
NLXC_PPD_INT  ON/OFF  [optional]
NLXC_DIVERGENCE_CORR ON/OFF [optional] 
%ENDBLOCK xc_definition

Weights for various forms should add up to 1.0 for both exchange and correlation.

Parameters with names that begin with NLXC_ apply to the way CASTEP calculates nonlocal exchange terms.

The NLXC_SCREENING_LENGTH parameter can be used to set the screening length value in Bohr^-1. Default value is 1.437 Å^-1 = 0.76 Bohr^-1.

NLXC_SCREENING_FUNCTION values can be set to either THOMAS-FERMI (default) or ERRORFUNCTION.

NLXC_DIVERGENCE_CORR can be set to either ON or OFF. Divergence correction is ON by default when using unscreened Hartree-Fock (HF) exchange, including such functionals as B3LYP or PBE0; and OFF by default when using screened HF exchange (functionals like SX-LDA, HSE03, HSE06).

NLXC_PPD_INT can be set to either ON or OFF. This setting invokes integration of Coulomb terms over a parallelepiped; default value is OFF for HF exchange and ON for screened HF.

It is not possible to set both NLXC_DIVERGENCE_CORR and NLXC_PPD_INT to ON simultaneously; CASTEP can only use one of these techniques at a time.

NLXC_DIVERGENCE_CORR can be applied only for insulators, when fixed occupancies are used.

Available options are:

LDA - Local Density Approximation
LDA-X - Local Density Approximation, exchange only
LDA-C - Local Density Approximation, correlation only
PW91 - Perdew Wang '91 GGA
PBE - Perdew Burke Ernzerhof GGA functional
PBE_X - Perdew Burke Ernzerhof GGA functional, exchange only
PBE_C - Perdew Burke Ernzerhof GGA functional, correlation only
RPBE - Revised Perdew Burke Ernzerhof GGA functional
WC - Wu-Cohen GGA functional
PBESOL - PBEsol, PBE functional for solids
PBESOL_X - PBEsol, PBE functional for solids, exchange only
PBESOL_C - PBEsol, PBE functional for solids, correlation only
B88_X - Becke (1988) GGA functional , exchange only
LYP_C - Lee-Yang-Parr (1988) GGA functional, correlation only
HF - exact exchange, no correlation
HF-LDA - exact exchange, LDA correlation
SX - screened exchange, no correlation
SX-LDA - screened exchange, LDA correlation
PBE0 - PBE0 hybrid functional
B3LYP - B3LYP hybrid functional
HSE03 - HSE03 hybrid functional
HSE06 - HSE06 hybrid functional

It is not possible to add more than one exact or screened exchange functional to the mix; for example, combining HF and PBE0 is not allowed. On the other hand, any number of local forms is allowed as long as the weights add up to unity.

Default

None

Example

The block below corresponds to the B3LYP exchange-correlation functional:

%BLOCK xc_definition
LDA-X   0.08
B88_X   0.72
LYP_C   0.81
LDA-C   0.19
HF      0.2
%ENDBLOCK xc_definition

XC_DEFINITION (.param)

Keyword type

Description

Default

Example

See Also: