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md_lj_le_module.f90
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md_lj_le_module.f90
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! md_lj_le_module.f90
! Force routine for MD, LJ atoms, Lees-Edwards boundaries
MODULE md_module
!------------------------------------------------------------------------------------------------!
! This software was written in 2016/17 !
! by Michael P. Allen <[email protected]>/<[email protected]> !
! and Dominic J. Tildesley <[email protected]> ("the authors"), !
! to accompany the book "Computer Simulation of Liquids", second edition, 2017 ("the text"), !
! published by Oxford University Press ("the publishers"). !
! !
! LICENCE !
! Creative Commons CC0 Public Domain Dedication. !
! To the extent possible under law, the authors have dedicated all copyright and related !
! and neighboring rights to this software to the PUBLIC domain worldwide. !
! This software is distributed without any warranty. !
! You should have received a copy of the CC0 Public Domain Dedication along with this software. !
! If not, see <http://creativecommons.org/publicdomain/zero/1.0/>. !
! !
! DISCLAIMER !
! The authors and publishers make no warranties about the software, and disclaim liability !
! for all uses of the software, to the fullest extent permitted by applicable law. !
! The authors and publishers do not recommend use of this software for any purpose. !
! It is made freely available, solely to clarify points made in the text. When using or citing !
! the software, you should not imply endorsement by the authors or publishers. !
!------------------------------------------------------------------------------------------------!
USE, INTRINSIC :: iso_fortran_env, ONLY : output_unit, error_unit
IMPLICIT NONE
PRIVATE
! Public routines
PUBLIC :: introduction, conclusion, allocate_arrays, deallocate_arrays
PUBLIC :: force
! Public data
INTEGER, PUBLIC :: n ! Number of atoms
REAL, DIMENSION(:,:), ALLOCATABLE, PUBLIC :: r ! Positions (3,n)
REAL, DIMENSION(:,:), ALLOCATABLE, PUBLIC :: v ! Velocities (3,n)
REAL, DIMENSION(:,:), ALLOCATABLE, PUBLIC :: f ! Forces (3,n)
! Private data
REAL, PARAMETER :: r_cut_sq = 2.0**(1.0/3.0), r_cut = SQRT(r_cut_sq) ! Minimum and cutoff of WCA LJ potential
! Public derived type
TYPE, PUBLIC :: potential_type ! A composite variable for interactions comprising
REAL :: pot ! the potential energy
REAL :: vir ! the virial and
REAL :: lap ! the Laplacian and
REAL :: pyx ! the off-diagonal virial part of the pressure tensor
LOGICAL :: ovr ! a flag indicating overlap (i.e. pot too high to use)
CONTAINS
PROCEDURE :: add_potential_type
GENERIC :: OPERATOR(+) => add_potential_type
END TYPE potential_type
CONTAINS
FUNCTION add_potential_type ( a, b ) RESULT (c)
IMPLICIT NONE
TYPE(potential_type) :: c ! Result is the sum of
CLASS(potential_type), INTENT(in) :: a, b ! the two inputs
c%pot = a%pot + b%pot
c%vir = a%vir + b%vir
c%pyx = a%pyx + b%pyx
c%lap = a%lap + b%lap
c%ovr = a%ovr .OR. b%ovr
END FUNCTION add_potential_type
SUBROUTINE introduction
IMPLICIT NONE
WRITE ( unit=output_unit, fmt='(a)' ) 'WCA shifted Lennard-Jones potential'
WRITE ( unit=output_unit, fmt='(a)' ) 'Diameter, sigma = 1'
WRITE ( unit=output_unit, fmt='(a)' ) 'Well depth, epsilon = 1'
WRITE ( unit=output_unit, fmt='(a)' ) 'Lees-Edwards boundaries'
END SUBROUTINE introduction
SUBROUTINE conclusion
IMPLICIT NONE
WRITE ( unit=output_unit, fmt='(a)') 'Program ends'
END SUBROUTINE conclusion
SUBROUTINE allocate_arrays ( box )
IMPLICIT NONE
REAL, INTENT(in) :: box ! Simulation box length
REAL :: r_cut_box
ALLOCATE ( r(3,n), v(3,n), f(3,n) )
r_cut_box = r_cut / box
IF ( r_cut_box > 0.5 ) THEN
WRITE ( unit=error_unit, fmt='(a,f15.6)' ) 'r_cut/box too large ', r_cut_box
STOP 'Error in allocate_arrays'
END IF
END SUBROUTINE allocate_arrays
SUBROUTINE deallocate_arrays
IMPLICIT NONE
DEALLOCATE ( r, v, f )
END SUBROUTINE deallocate_arrays
SUBROUTINE force ( box, strain, total )
IMPLICIT NONE
REAL, INTENT(in) :: box ! Simulation box length
REAL, INTENT(in) :: strain ! Shear strain
TYPE(potential_type), INTENT(out) :: total ! Composite of pot, vir, lap etc
! total%pot is the WCA LJ potential energy for whole system
! total%vir is the corresponding virial
! total%pyx is the virial part of the yx-element of the pressure tensor
! total%lap is the corresponding Laplacian
! total%ovr is a warning flag that there is an overlap
! This routine also calculates forces and stores them in the array f
! If total%ovr is set to .true., the forces etc should not be used
! It is assumed that positions are in units where box = 1
! Forces are calculated in units where sigma = 1 and epsilon = 1
! Lees-Edwards boundaries, in sliding brick arrangement
! Flow/gradient/vorticity directions are x/y/z == 1/2/3
INTEGER :: i, j
REAL :: r_cut_box, r_cut_box_sq, box_sq, rij_sq
REAL :: sr2, sr6, sr12
REAL, DIMENSION(3) :: rij, fij
REAL, PARAMETER :: sr2_ovr = 1.77 ! overlap threshold (pot > 100)
TYPE(potential_type) :: pair
r_cut_box = r_cut / box
r_cut_box_sq = r_cut_box ** 2
box_sq = box ** 2
IF ( r_cut_box > 0.5 ) THEN
WRITE ( unit=error_unit, fmt='(a,f15.6)' ) 'r_cut/box too large ', r_cut_box
STOP 'Error in force'
END IF
! Initialize
f = 0.0
total = potential_type ( pot=0.0, vir=0.0, pyx=0.0, lap=0.0, ovr=.FALSE. )
DO i = 1, n - 1 ! Begin outer loop over atoms
DO j = i + 1, n ! Begin inner loop over atoms
rij(:) = r(:,i) - r(:,j) ! Separation vector
rij(1) = rij(1) - ANINT ( rij(2) ) * strain ! Extra correction in box=1 units
rij(:) = rij(:) - ANINT ( rij(:) ) ! Periodic boundary conditions in box=1 units
rij_sq = SUM ( rij**2 ) ! Squared separation
IF ( rij_sq < r_cut_box_sq ) THEN ! Check within cutoff
rij_sq = rij_sq * box_sq ! Now in sigma=1 units
rij(:) = rij(:) * box ! Now in sigma=1 units
sr2 = 1.0 / rij_sq ! (sigma/rij)**2
pair%ovr = sr2 > sr2_ovr ! Overlap if too close
sr6 = sr2 ** 3
sr12 = sr6 ** 2
pair%pot = sr12 - sr6 ! LJ pair potential (cut but not shifted)
pair%vir = pair%pot + sr12 ! LJ pair virial
pair%pot = pair%pot + 0.25 ! WCA LJ pair potential (cut-and-shifted)
pair%lap = ( 22.0*sr12 - 5.0*sr6 ) * sr2 ! LJ pair Laplacian
fij = rij * pair%vir * sr2 ! LJ pair forces
pair%pyx = rij(2)*fij(1) ! Off-diagonal element of pressure tensor
total = total + pair
f(:,i) = f(:,i) + fij
f(:,j) = f(:,j) - fij
END IF ! End check within cutoff
END DO ! End inner loop over atoms
END DO ! End outer loop over atoms
! Multiply results by numerical factors
f = f * 24.0 ! 24*epsilon
total%pot = total%pot * 4.0 ! 4*epsilon
total%vir = total%vir * 24.0 / 3.0 ! 24*epsilon and divide virial by 3
total%pyx = total%pyx * 24.0 ! 24*epsilon
total%lap = total%lap * 24.0 * 2.0 ! 24*epsilon and factor 2 for ij and ji
END SUBROUTINE force
END MODULE md_module