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md_lj_vl_module.f90
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md_lj_vl_module.f90
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! md_lj_vl_module.f90
! Force routine for MD, LJ atoms, using Verlet neighbour list
MODULE md_module
!------------------------------------------------------------------------------------------------!
! This software was written in 2016/17 !
! by Michael P. Allen <m.p.allen@warwick.ac.uk>/<m.p.allen@bristol.ac.uk> !
! and Dominic J. Tildesley <d.tildesley7@gmail.com> ("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, hessian
! 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)
! Public derived type
TYPE, PUBLIC :: potential_type ! A composite variable for interactions comprising
REAL :: cut ! the potential energy cut (but not shifted) at r_cut and
REAL :: pot ! the potential energy cut-and-shifted at r_cut and
REAL :: vir ! the virial and
REAL :: lap ! the Laplacian and
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%cut = a%cut + b%cut
c%pot = a%pot + b%pot
c%vir = a%vir + b%vir
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)' ) 'Lennard-Jones potential'
WRITE ( unit=output_unit, fmt='(a)' ) 'Cut-and-shifted version for dynamics'
WRITE ( unit=output_unit, fmt='(a)' ) 'Cut (but not shifted) version also calculated'
WRITE ( unit=output_unit, fmt='(a)' ) 'Diameter, sigma = 1'
WRITE ( unit=output_unit, fmt='(a)' ) 'Well depth, epsilon = 1'
END SUBROUTINE introduction
SUBROUTINE conclusion
IMPLICIT NONE
WRITE ( unit=output_unit, fmt='(a)') 'Program ends'
END SUBROUTINE conclusion
SUBROUTINE allocate_arrays ( box, r_cut )
USE verlet_list_module, ONLY : initialize_list
IMPLICIT NONE
REAL, INTENT(in) :: box ! Simulation box length
REAL, INTENT(in) :: r_cut ! Potential cutoff distance
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
CALL initialize_list ( n, r_cut_box )
END SUBROUTINE allocate_arrays
SUBROUTINE deallocate_arrays
USE verlet_list_module, ONLY : finalize_list
IMPLICIT NONE
DEALLOCATE ( r, v, f )
CALL finalize_list
END SUBROUTINE deallocate_arrays
SUBROUTINE force ( box, r_cut, total )
USE verlet_list_module, ONLY : point, list, make_list
IMPLICIT NONE
REAL, INTENT(in) :: box ! Simulation box length
REAL, INTENT(in) :: r_cut ! Potential cutoff distance
TYPE(potential_type), INTENT(out) :: total ! Composite of pot, vir, lap etc
! total%pot is the nonbonded cut-and-shifted potential energy for whole system
! total%cut is the nonbonded cut (but not shifted) potential energy for whole system
! total%vir is the corresponding virial
! 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
! Forces are derived from pot, not cut (which has a discontinuity)
! 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
! Uses a Verlet list
INTEGER :: i, j, k
REAL :: r_cut_box, r_cut_box_sq, box_sq, rij_sq
REAL :: sr2, sr6, sr12, pot_cut
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
! Calculate potential at cutoff
sr2 = 1.0 / r_cut**2 ! in sigma=1 units
sr6 = sr2 ** 3
sr12 = sr6 **2
pot_cut = sr12 - sr6 ! Without numerical factor 4
CALL make_list ( n, r )
! Initialize
f = 0.0
total = potential_type ( pot=0.0, cut=0.0, vir=0.0, lap=0.0, ovr=.FALSE. )
DO i = 1, n - 1 ! Begin outer loop over atoms
DO k = point(i), point(i+1) - 1 ! Begin inner loop over neighbour atoms (if any)
j = list(k) ! Neighbour atom index
rij(:) = r(:,i) - r(:,j) ! Separation vector
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
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%cut = sr12 - sr6 ! LJ pair potential (cut but not shifted)
pair%vir = pair%cut + sr12 ! LJ pair virial
pair%pot = pair%cut - pot_cut ! 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
total = total + pair
f(:,i) = f(:,i) + fij
f(:,j) = f(:,j) - fij
END IF
END DO ! End inner loop over neighbour atoms (if any)
END DO ! End outer loop over atoms
! Multiply results by numerical factors
f = f * 24.0 ! 24*epsilon
total%cut = total%cut * 4.0 ! 4*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%lap = total%lap * 24.0 * 2.0 ! 24*epsilon and factor 2 for ij and ji
END SUBROUTINE force
FUNCTION hessian ( box, r_cut ) RESULT ( hes )
USE verlet_list_module, ONLY : point, list, make_list
IMPLICIT NONE
REAL :: hes ! Returns total Hessian
REAL, INTENT(in) :: box ! Simulation box length
REAL, INTENT(in) :: r_cut ! Potential cutoff distance
! Calculates Hessian function (for 1/N correction to config temp)
! This routine is only needed in a constant-energy ensemble
! It is assumed that positions are in units where box = 1
! but the result is given in units where sigma = 1 and epsilon = 1
! It is assumed that forces have already been calculated
! Uses a Verlet list
INTEGER :: i, j, k
REAL :: r_cut_box, r_cut_box_sq, box_sq, rij_sq
REAL :: sr2, sr6, sr8, sr10, rf, ff, v1, v2
REAL, DIMENSION(3) :: rij, fij
r_cut_box = r_cut / box
r_cut_box_sq = r_cut_box ** 2
box_sq = box ** 2
CALL make_list ( n, r )
hes = 0.0
DO i = 1, n - 1 ! Begin outer loop over atoms
DO k = point(i), point(i+1) - 1 ! Begin inner loop over neighbour atoms (if any)
j = list(k) ! Neighbour atom index
rij(:) = r(:,i) - r(:,j) ! Separation vector
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
rij_sq = rij_sq * box_sq ! Now in sigma=1 units
rij(:) = rij(:) * box ! Now in sigma=1 units
fij(:) = f(:,i) - f(:,j) ! Difference in forces
ff = DOT_PRODUCT(fij,fij)
rf = DOT_PRODUCT(rij,fij)
sr2 = 1.0 / rij_sq
sr6 = sr2 ** 3
sr8 = sr6 * sr2
sr10 = sr8 * sr2
v1 = 24.0 * ( 1.0 - 2.0 * sr6 ) * sr8
v2 = 96.0 * ( 7.0 * sr6 - 2.0 ) * sr10
hes = hes + v1 * ff + v2 * rf**2
END IF
END DO ! End inner loop over neighbour atoms (if any)
END DO ! End outer loop over atoms
END FUNCTION hessian
END MODULE md_module