Developers:Xc module
From OctopusWiki
Contents |
Introduction
The module xc takes care of most of the problem of calculating the exchange and correlation potential and energy functionals. Unfortunately, it cannot handle all the problem, as it would be desirable, because the OEP functionals have to be handled within the hamiltonian module (to obtain an OEP functional one needs to apply the Hamiltonian). Since Fortran 90 is unable of handling circular dependencies, the obtention of OEP functionals (or the obtention of the SIC through the OEP method) is done in the h_xc_oep subroutine in the hamiltonian module. We can consider h_xc_oep to be within the xc module in spirit.
Types
xc_type
type(xc_functl_type) :: functl(2) integer :: sic_correction type(xc_functl_type) :: sic_aux(2) integer :: mGGA_implementation integer :: oep_level FLOAT :: oep_mixing logical :: nlcc
This data type holds all the necessary information to fully characterize the exchange and correlation functional used in an octopus calculation. Its componets are the following:
- functl(2): The first element of this two-element array is the xc_functl_type structure that holds the information about the exchange functional; the second element holds the correlation functional.
- sic_correction determines wether self-interaction corrections (SIC) are to be applied or not. It only holds if any of the functl components is holding a functional of the XC_FAMILY_LDA or XC_FAMILY_GGA families (see @ref{type xc_functl_type}). The possible values are: 0 (do not apply SIC corrections), or anything else
(do apply SIC corrections).
- sic_aux(2): These are two xc_functl_type structures, which hold auxiliary exchange and correlation functionals, necessary to builid the SIC. [WARNING: I believe that this could be avoided]
- mGGA_implementation: The mGGA functional is in fact an orbital dependent functional. It may then also use the OEP formalism. The precise way in which the mGGA functional is implemented is then decided by this variable, which may take two values:
- mGGA_implementation = 1: GEA-like implementation (not working).
- mGGA_implementation = 2: OEP-like implementation (not yet implemented).
- oep_level: The optimized effective potential method is applied in three cases: (i) in case a SIC is applied to a LDA or GGA functional; (ii) in case the exact exchange functional is requested; and (iii) in case the mGGA is to be implemented through the OEP (mGGA_implementation = 2). However, several possible choices are then possible to approximate (or calculate exactly) the OEP. The options - the values that the integer oep_level may take - are listed in @ref{OEP levels}.
- oep_mixing
- nlcc
Constants
OEP levels
- XC_OEP_NONE = 0
- XC_OEP_SLATER = 1
- XC_OEP_KLI = 2
- XC_OEP_CEDA = 3
- XC_OEP_FULL = 4
Subroutines
xc_init
type(xc_type), intent(out) :: xcs logical, intent(in) :: nlcc integer, intent(in) :: spin_channels
This subroutine initializes a xc_type variable, the xcs argument, maiinly by reading in variables from the input file. Two input arguments need, however, to be passed in:
- nlcc, used to fill in xc_type%nlcc (see type xc_type).
- spin_channels, which is needed to call xc_functl_init_exchange and xc_functl_init_correlation (see xc_functl). It should be equal to one if the calculations are unpolarized, and equal to two if the calculations are spin-polarized, or based in two-spinors.
This routine is in charge of calling exc_functl_init_exchange and xc_functl_init_correlation.
It also reads some extra variables from the input file:
- MGGAimplementation: it is used to fill xcs%mGGA_implementation.
- SICCorrection: used to fill xcs%sic_correction.
- OEP_level: used to fill xcs%oep_level.
- OEP_mixing: used to fill xcs%oep_mixing.
xc_write_info
type(xc_type), intent(in) :: xcs integer, intent(in) :: iunit
xc_end
type(xc_type), intent(inout) :: xcs
subroutine xc_get_vxc
type(xc_type), intent(in) :: xcs type(mesh_type), intent(in) :: m type(f_der_type), intent(inout) :: f_der type(states_type), intent(in) :: st FLOAT, intent(inout) :: vxc(:,:), ex, ec FLOAT, intent(in) :: ip, qtot logical, optional, intent(in) :: aux
This routine fills the exchange and correlation potential vxc (as well as the exchange and correlation energies, ex and ec. It takes care of the "simple" functionals, i.e. LDA, GGA and mGGA; The SIC corrections to these functionals or the exact exchange OEP is handled in the h_xc_OEP subroutine, which should be called after xc_get_vxc.
The input variables are:
- xcs: This is the xc_type variable that holds all the information about the exchange and correlation functional. It should have been initialized previously by the xc_init subroutine.
- m: The mesh where all the system functions live.
- f_der: This is the f_der_type structure: the structure that knows how to do the derivatives. It is defined inout for technical reasons, but it should not change.
- st: The states_type structure that holds the density and the Kohn-Sham states.
- ip: The ionization potential of the system, or some reasonable estimation for it, necessary only in case the van Leeuween & Baerends potential is to be used.
- qtot: The total electron number of the system, also only necessary in case the van Leeuween & Baerends potential is to be used.
- aux: This is an optional flag:
- If it is passed, and it is set to .true., the exchange and correlation functional used will be those held in xcs%sic_aux. This is done in order to calculate the self-interaction corrections.
- Otherwise, they will be those held in xcs%functl.
- vxc, ex, ec: the values of these three variables on entry is added to the calculated values.
The output variables are:
- ex: The exchange energy.
- ec: The correlation energy.
- vxc(:, :): The dimension of this array are (m%np, st%d%nspin). On output,
it holds the exchange and correlation potential - added to whatever it held before.
