Implement SH with Induced Transitions method

src/sh_integ.F90

@@ -10,6 +10,7 @@ module mod_sh_integ
    public :: sh_set_initialwf, sh_set_energy_shift
    public :: sh_select_integrator, sh_integrate_wf
    public :: sh_decoherence_correction, check_popsum, sh_TFS_transmat
+   public :: shwit_switch
 
    ! Restart functionality
    public :: sh_write_phase_bin, sh_read_phase_bin
@@ -445,6 +446,20 @@ subroutine sh_set_initialwf(initial_state)
       cel_re(initial_state) = 1.0D0
    end subroutine sh_set_initialwf
 
+   ! A horrible hack for SHwIT, we just switch
+   ! switch the coefficients between S1-S0
+   ! TODO: Do something less gross
+   subroutine shwit_switch()
+      real(DP) :: tmp
+      tmp = cel_re(1)
+      cel_re(1) = cel_re(2)
+      cel_re(2) = tmp
+
+      tmp = cel_im(1)
+      cel_im(1) = cel_im(2)
+      cel_im(2) = tmp
+   end subroutine shwit_switch
+
    subroutine sh_set_energy_shift(potential_energy)
       real(DP), intent(in) :: potential_energy
 

src/surfacehop.F90

@@ -77,7 +77,12 @@ module mod_sh
    ! Stop simulation if total energy drift since the beginning exceeds energydriftthr
    real(DP) :: energydriftthr = 1.0D0
 
-   ! Special case for adiabatic TDDFT, terminate when close to S1-S0 crossing
+   ! Surface Hopping with Induced Transitions,
+   ! typically used with methods that can't describe S1S0 intersections,
+   ! such as TDDFT or ADC2.
+   ! In this case, we force a hop to S0 when the energy difference
+   ! between S1 and S0 drops below user defined threshold.
+   logical :: SHwIT = .false.
    real(DP) :: dE_S0S1_thr = 0.0D0 !eV
    ! NA Couplings
    real(DP), allocatable :: nacx(:, :, :), nacy(:, :, :), nacz(:, :, :)
@@ -100,7 +105,7 @@ module mod_sh
 
    namelist /sh/ istate_init, nstate, substep, deltae, integ, couplings, nohop, phase, decoh_alpha, popthr, ignore_state, &
       nac_accu1, nac_accu2, popsumthr, energydifthr, energydriftthr, velocity_rescaling, revmom, &
-      dE_S0S1_thr, correct_decoherence
+      shwit, dE_S0S1_thr, correct_decoherence
    save
 
 contains
@@ -252,7 +257,7 @@ subroutine check_sh_parameters()
          write (stdout, '(A)') 'Using approximate Baeck-An couplings.'
       case ('none')
          inac = 2
-         write (stdout, '(A)') 'Ignoring nonadaiabatic couplings.'
+         write (stdout, '(A)') 'Ignoring nonadiabatic couplings.'
       case default
          write (stderr, '(A)') 'Parameter "couplings" must be "analytic", "baeck-an" or "none".'
          error = .true.
@@ -279,7 +284,7 @@ subroutine check_sh_parameters()
          error = .true.
       end if
 
-      if (inac == 2 .and. nohop == 0) then
+      if (inac == 2 .and. nohop == 0 .and. .not. shwit) then
          write (stdout, '(A)') 'WARNING: For simulations without couplings(="none") hopping probability cannot be determined.'
          nohop = 1
       end if
@@ -821,6 +826,12 @@ subroutine surfacehop(x, y, z, vx, vy, vz, vx_old, vy_old, vz_old, dt, eclas)
          !itp loop
       end do
 
+      if (SHwIT) then
+         call shwit_check(en_array(1), en_array(2), dE_S0S1_thr, &
+               vx, vy, vz, eclas)
+      end if
+
+
       ! set tocalc array for the next step
       call set_tocalc()
 
@@ -1211,15 +1222,6 @@ subroutine check_energy(vx_old, vy_old, vz_old, vx, vy, vz)
       real(DP), intent(in) :: vx_old(:, :), vy_old(:, :), vz_old(:, :)
       real(DP) :: ekin, ekin_old, entot, entot_old
 
-      ! Special case for running AIMD with TDDFT:
-      ! End the simulation when S1-S0 energy difference drops
-      ! below a certain small threshold.
-      if (dE_S0S1_thr > 0.0D0 .and. nstate >= 2) then
-         call check_S0S1_gap(en_S0=en_array(1), &
-                             en_S1=en_array(2), &
-                             threshold_ev=dE_S0S1_thr)
-      end if
-
       ekin = ekin_v(vx, vy, vz)
       ekin_old = ekin_v(vx_old, vy_old, vz_old)
 
@@ -1242,26 +1244,29 @@ subroutine check_energy(vx_old, vy_old, vz_old, vx, vy, vz)
 
    end subroutine check_energy
 
-   subroutine check_S0S1_gap(en_S0, en_S1, threshold_ev)
-      use mod_general, only: STOP_SIMULATION
+   subroutine shwit_check(en_S0, en_S1, threshold_ev, vx, vy, vz, eclas)
       use mod_const, only: AUtoEV
       real(DP), intent(in) :: en_S0, en_S1
       real(DP), intent(in) :: threshold_ev
+      real(DP), intent(inout) :: vx(:, :), vy(:, :), vz(:, :), eclas
       real(DP) :: dE_S0S1
 
+      ! We only hop from S1 to S0
+      if (istate /= 2) return
+
       dE_S0S1 = (en_S1 - en_S0) * AUtoEV
 
       if (dE_S0S1 < threshold_ev) then
-         write (stdout, *) 'S1 - S0 gap dropped below threshold!'
+         write (stdout, *) 'SHwIT: S1 - S0 gap dropped below threshold!'
          write (stdout, *) dE_S0S1, ' < ', threshold_ev
-         ! TODO: This is an unfortunate hack,
-         ! but we can't directly stop here since we need the last step
-         ! to be written to the output files.
-         write (stdout, *) 'Simulation will stop at the end of this step.'
+         write (stdout, *) "Let's jump to S0 and continue!"
          ! This global flag is checked in the main loop in abin.F90
-         STOP_SIMULATION = .true.
+         ! STOP_SIMULATION = .true.
+         call try_hop_simple_rescale(vx, vy, vz, 2, 1, eclas)
+         ! Horrible hack to exchange c_el coefficients between S1 - S0
+         call shwit_switch()
       end if
-   end subroutine check_S0S1_gap
+   end subroutine shwit_check
 
    subroutine check_energydrift(vx, vy, vz)
       use mod_const, only: AUtoEV

tests/SH_S0S1/input.in

@@ -19,17 +19,18 @@ inose=0,
 /
 
 &sh
+shwit=.true.
 dE_S0S1_thr=9.0
-istate_init=3,
+istate_init=2,
 nstate=3,
 substep=100,
 deltae=100.,
 integ='butcher',
-couplings='analytic',	! non-adiabatic coupling terms 'analytic', 'baeck-an', 'none'
+couplings='none',	! non-adiabatic coupling terms 'analytic', 'baeck-an', 'none'
 nohop=0,
 decoh_alpha=0.1
 popthr=0.01
-energydifthr=0.50
-energydriftthr=0.50
+energydifthr=1.00
+energydriftthr=1.00
 correct_decoherence=.false.
 /