!--------------------------------------------------------------------------------------------------!
!   CP2K: A general program to perform molecular dynamics simulations                              !
!   Copyright (C) 2000 - 2018  CP2K developers group                                               !
!--------------------------------------------------------------------------------------------------!

! **************************************************************************************************
!> \brief Methods to include the effect of an external potential during an MD
!>        or energy calculation
!> \author Teodoro Laino (03.2008) [tlaino]
! **************************************************************************************************
MODULE external_potential_methods
   USE cp_log_handling,                 ONLY: cp_get_default_logger,&
                                              cp_logger_type,&
                                              cp_to_string
   USE cp_subsys_types,                 ONLY: cp_subsys_get,&
                                              cp_subsys_type
   USE force_env_types,                 ONLY: force_env_get,&
                                              force_env_set,&
                                              force_env_type
   USE force_fields_util,               ONLY: get_generic_info
   USE fparser,                         ONLY: evalf,&
                                              evalfd,&
                                              finalizef,&
                                              initf,&
                                              parsef
   USE input_section_types,             ONLY: section_vals_get,&
                                              section_vals_get_subs_vals,&
                                              section_vals_type,&
                                              section_vals_val_get
   USE kinds,                           ONLY: default_path_length,&
                                              default_string_length,&
                                              dp
   USE memory_utilities,                ONLY: reallocate
   USE particle_list_types,             ONLY: particle_list_type
   USE string_utilities,                ONLY: compress
#include "./base/base_uses.f90"

   IMPLICIT NONE

   PRIVATE
   CHARACTER(len=*), PARAMETER, PRIVATE :: moduleN = 'external_potential_methods'
   PUBLIC :: add_external_potential

CONTAINS

! **************************************************************************************************
!> \brief ...
!> \param force_env ...
!> \date 03.2008
!> \author Teodoro Laino - University of Zurich [tlaino]
! **************************************************************************************************
   SUBROUTINE add_external_potential(force_env)
      TYPE(force_env_type), POINTER                      :: force_env

      CHARACTER(len=*), PARAMETER :: routineN = 'add_external_potential', &
         routineP = moduleN//':'//routineN

      CHARACTER(LEN=default_path_length)                 :: coupling_function
      CHARACTER(LEN=default_string_length)               :: def_error, this_error
      CHARACTER(LEN=default_string_length), &
         DIMENSION(:), POINTER                           :: my_par
      INTEGER                                            :: a_var, handle, i, iatom, j, k, n_var, &
                                                            natom, rep
      INTEGER, DIMENSION(:), POINTER                     :: iatms, nparticle
      LOGICAL                                            :: useall
      REAL(KIND=dp)                                      :: dedf, dx, energy, err, lerr
      REAL(KIND=dp), DIMENSION(:), POINTER               :: my_val
      TYPE(cp_logger_type), POINTER                      :: logger
      TYPE(cp_subsys_type), POINTER                      :: subsys
      TYPE(particle_list_type), POINTER                  :: particles
      TYPE(section_vals_type), POINTER                   :: ext_pot_section

      useall = .FALSE.
      CALL timeset(routineN, handle)
      NULLIFY (my_par, my_val, logger, subsys, particles, ext_pot_section, nparticle)
      ext_pot_section => section_vals_get_subs_vals(force_env%force_env_section, &
                                                    "EXTERNAL_POTENTIAL")
      CALL section_vals_get(ext_pot_section, n_repetition=n_var)
      DO rep = 1, n_var
         natom = 0
         logger => cp_get_default_logger()
         CALL section_vals_val_get(ext_pot_section, "DX", r_val=dx, i_rep_section=rep)
         CALL section_vals_val_get(ext_pot_section, "ERROR_LIMIT", r_val=lerr, i_rep_section=rep)
         CALL get_generic_info(ext_pot_section, "FUNCTION", coupling_function, my_par, my_val, &
                               input_variables=(/"X", "Y", "Z"/), i_rep_sec=rep)
         CALL initf(1)
         CALL parsef(1, TRIM(coupling_function), my_par)

         ! Apply potential on all atoms, computing energy and forces
         NULLIFY (particles, subsys)
         CALL force_env_get(force_env, subsys=subsys)
         CALL cp_subsys_get(subsys, particles=particles)
         CALL force_env_get(force_env, additional_potential=energy)
         CALL section_vals_val_get(ext_pot_section, "ATOMS_LIST", n_rep_val=a_var, i_rep_section=rep)
         DO k = 1, a_var
            CALL section_vals_val_get(ext_pot_section, "ATOMS_LIST", i_rep_val=k, i_vals=iatms, i_rep_section=rep)
            CALL reallocate(nparticle, 1, natom+SIZE(iatms))
            nparticle(natom+1:natom+SIZE(iatms)) = iatms
            natom = natom+SIZE(iatms)
         END DO
         IF (a_var == 0) THEN
            natom = particles%n_els
            useall = .TRUE.
         END IF
         DO i = 1, natom
            IF (useall) THEN
               iatom = i
            ELSE
               iatom = nparticle(i)
            END IF
            my_val(1) = particles%els(iatom)%r(1)
            my_val(2) = particles%els(iatom)%r(2)
            my_val(3) = particles%els(iatom)%r(3)

            energy = energy+evalf(1, my_val)
            DO j = 1, 3
               dedf = evalfd(1, j, my_val, dx, err)
               IF (ABS(err) > lerr) THEN
                  WRITE (this_error, "(A,G12.6,A)") "(", err, ")"
                  WRITE (def_error, "(A,G12.6,A)") "(", lerr, ")"
                  CALL compress(this_error, .TRUE.)
                  CALL compress(def_error, .TRUE.)
                  CALL cp_warn(__LOCATION__, &
                               'ASSERTION (cond) failed at line '//cp_to_string(__LINE__)// &
                               ' Error '//TRIM(this_error)//' in computing numerical derivatives larger then'// &
                               TRIM(def_error)//' .')
               END IF
               particles%els(iatom)%f(j) = particles%els(iatom)%f(j)-dedf
            END DO
         END DO
         CALL force_env_set(force_env, additional_potential=energy)
         DEALLOCATE (my_par)
         DEALLOCATE (my_val)
         IF (a_var /= 0) THEN
            DEALLOCATE (nparticle)
         END IF
         CALL finalizef()
      END DO
      CALL timestop(handle)
   END SUBROUTINE add_external_potential

END MODULE external_potential_methods
