Custom types for holding simulation state

[danielhollas]

Maybe something like this

  type :: energies
     ! Ab initio electronic energy
     real(DP) :: e_pot
     ! Harmonic energy of the PI bead necklace
     real(DP) :: e_pi
  end type

  type :: forces
     ! Ab initio forces
     real(DP), allocatable, dimension(:, :) :: fxc, fyc, fzc
     ! Harmonic PI forces
     real(DP), allocatable, dimension(:, :) :: fxq, fyq, fzq
     ! Difference forces between full and reference potential
     real(DP), allocatable, dimension(:, :) :: fxdiff, fydiff, fzdiff
  end type

  type :: positions
     real(DP), allocatable, dimension(:, :) ::x, y, z
  end type

  type :: momenta
     real(DP), allocatable, dimension(:, :) ::px, py, pz
  end type

  type :: electronic_structure
     ! Dipoles, Transition Dipoles
     ! NAC couplings?
     real(DP), allocatable, dimension(:) :: dip, tdip
  end type

  type mdstate
     type(energies) :: e
     type(forces) :: f
     type(momenta) :: p
     type(positions) :: q
     type(electronic_structure) :: elstruct
  end type

  type(mdstate) :: state, state_prev

For surfaceHopping, we could then easily hold the whole state from a previous step, and be able to just do

state_prev = prev

or vice versa in case we wanted to implement adaptive time step. See also https://stackoverflow.com/questions/19111471/fortran-derived-type-assignment