Line data    Source code

       1             : !--------------------------------------------------------------------------------------------------!
       2             : !   CP2K: A general program to perform molecular dynamics simulations                              !
       3             : !   Copyright 2000-2024 CP2K developers group <https://cp2k.org>                                   !
       4             : !                                                                                                  !
       5             : !   SPDX-License-Identifier: GPL-2.0-or-later                                                      !
       6             : !--------------------------------------------------------------------------------------------------!
       7             : 
       8             : ! **************************************************************************************************
       9             : !> \par History
      10             : !>      15.10.2007 Giovanni Bussi - Implementation validated.
      11             : !> \author Teodoro Laino - 09.2007 - University of Zurich [tlaino]
      12             : ! **************************************************************************************************
      13             : MODULE csvr_system_utils
      14             : 
      15             :    USE kinds,                           ONLY: dp
      16             :    USE parallel_rng_types,              ONLY: rng_stream_type
      17             : #include "./base/base_uses.f90"
      18             : 
      19             :    IMPLICIT NONE
      20             : 
      21             :    PRIVATE
      22             : 
      23             :    LOGICAL, PARAMETER                   :: debug_this_module = .FALSE.
      24             :    PUBLIC                               :: rescaling_factor
      25             :    CHARACTER(len=*), PARAMETER, PRIVATE :: moduleN = 'csvr_system_utils'
      26             : 
      27             : CONTAINS
      28             : 
      29             : ! **************************************************************************************************
      30             : !> \brief Stochastic velocity rescale, as described in
      31             : !>      Bussi, Donadio and Parrinello, J. Chem. Phys. 126, 014101 (2007)
      32             : !>
      33             : !>      This subroutine implements Eq.(A7) and returns the new value for the kinetic energy,
      34             : !>      which can be used to rescale the velocities.
      35             : !>      The procedure can be applied to all atoms or to smaller groups.
      36             : !>      If it is applied to intersecting groups in sequence, the kinetic energy
      37             : !>      that is given as an input (kk) has to be up-to-date with respect to the previous
      38             : !>      rescalings.
      39             : !>
      40             : !>      When applied to the entire system, and when performing standard molecular dynamics
      41             : !>      (fixed c.o.m. (center of mass))
      42             : !>      the degrees of freedom of the c.o.m. have to be discarded in the calculation of ndeg,
      43             : !>      and the c.o.m. momentum HAS TO BE SET TO ZERO.
      44             : !>      When applied to subgroups, one can chose to:
      45             : !>      (a) calculate the subgroup kinetic energy in the usual reference frame, and count
      46             : !>          the c.o.m. in ndeg
      47             : !>      (b) calculate the subgroup kinetic energy with respect to its c.o.m. motion, discard
      48             : !>          the c.o.m. in ndeg and apply the rescale factor with respect to the subgroup c.o.m.
      49             : !>          velocity.
      50             : !>      They should be almost equivalent.
      51             : !>      If the subgroups are expected to move one respect to the other, the choice (b)
      52             : !>      should be better.
      53             : !>
      54             : !>      If a null relaxation time is required (taut=0.0), the procedure reduces to an istantaneous
      55             : !>      randomization of the kinetic energy, as described in paragraph IIA.
      56             : !>
      57             : !>      HOW TO CALCULATE THE EFFECTIVE-ENERGY DRIFT
      58             : !>      The effective-energy (htilde) drift can be used to check the integrator against
      59             : !>      discretization errors.
      60             : !>      The easiest recipe is:
      61             : !>      htilde = h + conint
      62             : !>      where h is the total energy (kinetic + potential)
      63             : !>      and conint is a quantity accumulated along the trajectory as minus the sum of all
      64             : !>      the increments of kinetic energy due to the thermostat.
      65             : !>
      66             : !>      Variables:
      67             : !>       kk    ! present value of the kinetic energy of the atoms to be thermalized (in arbitrary units)
      68             : !>       sigma ! target average value of the kinetic energy (ndeg k_b T/2)  (in the same units as kk)
      69             : !>       ndeg  ! number of degrees of freedom of the atoms to be thermalized
      70             : !>       taut  ! relaxation time of the thermostat, in units of 'how often this routine is called'
      71             : !> \param kk ...
      72             : !> \param sigma ...
      73             : !> \param ndeg ...
      74             : !> \param taut ...
      75             : !> \param rng_stream ...
      76             : !> \return ...
      77             : !> \date 09.2007
      78             : !> \author Giovanni Bussi - ETH Zurich, Lugano 10.2007
      79             : ! **************************************************************************************************
      80      114284 :    FUNCTION rescaling_factor(kk, sigma, ndeg, taut, rng_stream) RESULT(my_res)
      81             :       REAL(KIND=dp), INTENT(IN)                          :: kk, sigma
      82             :       INTEGER, INTENT(IN)                                :: ndeg
      83             :       REAL(KIND=dp), INTENT(IN)                          :: taut
      84             :       TYPE(rng_stream_type), INTENT(INOUT)               :: rng_stream
      85             :       REAL(KIND=dp)                                      :: my_res
      86             : 
      87             :       REAL(KIND=dp)                                      :: factor, resample, reverse, rr
      88             : 
      89      114284 :       my_res = 0.0_dp
      90      114284 :       IF (kk > 0.0_dp) THEN
      91      114186 :          IF (taut > 0.1_dp) THEN
      92      114186 :             factor = EXP(-1.0_dp/taut)
      93             :          ELSE
      94             :             factor = 0.0_dp
      95             :          END IF
      96      114186 :          rr = rng_stream%next()
      97      114186 :          reverse = 1.0_dp
      98             :          ! reverse of momentum is implemented to have the correct limit to Langevin dynamics for ndeg=1
      99             :          ! condition: rr < -SQRT(ndeg*kk*factor/(sigma*(1.0_dp-factor)))
     100      114186 :          IF ((rr*rr*sigma*(1.0_dp - factor)) > (ndeg*kk*factor) .AND. rr <= 0.0_dp) reverse = -1.0_dp
     101             :          ! for ndeg/=1, the reverse of momentum is not necessary. in principles, it should be there.
     102             :          ! in practice, it is better to skip it to avoid unnecessary slowing down of the dynamics in the small taut regime
     103             :          ! anyway, this should not affect the final ensemble
     104      114186 :          IF (ndeg /= 1) reverse = 1.0_dp
     105             :          resample = kk + (1.0_dp - factor)*(sigma*(sumnoises(ndeg - 1, rng_stream) + rr**2)/REAL(ndeg, KIND=dp) - kk) &
     106      114186 :                     + 2.0_dp*rr*SQRT(kk*sigma/ndeg*(1.0_dp - factor)*factor)
     107             : 
     108      114186 :          resample = MAX(0.0_dp, resample)
     109      114186 :          my_res = reverse*SQRT(resample/kk)
     110             :       END IF
     111      114284 :    END FUNCTION rescaling_factor
     112             : 
     113             : ! **************************************************************************************************
     114             : !> \brief returns the sum of n independent gaussian noises squared
     115             : !>      (i.e. equivalent to summing the square of the return values of nn calls to gasdev)
     116             : !> \param nn ...
     117             : !> \param rng_stream ...
     118             : !> \return ...
     119             : !> \date 09.2007
     120             : !> \author Teo - University of Zurich
     121             : ! **************************************************************************************************
     122      114186 :    FUNCTION sumnoises(nn, rng_stream) RESULT(sum_gauss)
     123             :       INTEGER, INTENT(IN)                                :: nn
     124             :       TYPE(rng_stream_type), INTENT(INOUT)               :: rng_stream
     125             :       REAL(KIND=dp)                                      :: sum_gauss
     126             : 
     127             :       INTEGER                                            :: i
     128             : 
     129      114186 :       sum_gauss = 0.0_dp
     130     3694152 :       DO i = 1, nn
     131     3694152 :          sum_gauss = sum_gauss + rng_stream%next()**2
     132             :       END DO
     133             : 
     134      114186 :    END FUNCTION sumnoises
     135             : 
     136             : END MODULE csvr_system_utils