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FermionHamiltonian

Before doing calculation of a finite size fermion system, one need to define a Hamiltonian object. This object contains all the information of Hamiltonian in the cluster, like : hopping terms, Coulomb U, inter site V....

After it being defined, it can futher be used for Lanczos or Hoseholder method directly.

In this example, adding a hopping term between site 0 and site 3 is shown. In formalism, it represent the term:

t₀₃Σ_σ(c⁺_0σc_3σ+h.c.)

one can find many more kinds of interactions in source code directly.

program main
  use FermionHamiltonian
  implicit none

  TYPE(Ham)::h
  integer::hpara(8)
  complex*16::v
  character*16::Intp


  ! Initialization  (needed)
  call h%Initialization( ns = 6)


  ! start append H terms  (needed)
  call h%StartAppendingInteraction()


  ! append a hopping term between site 0 and site 3
  Intp     = "Hopping"
  hpara(1) = 0
  hpara(2) = 3
  v        = (1.0_8,0._8)

  call h%AppendingInteraction(InterType=Intp,InterPara=hpara,InterV=v)


  !End appending (needed)  After this subroutine being called, no more terms can be appended into H
  call h%EndAppendingInteraction()


  ! more terms can be found in source code directly.
endprogram

function h%StartAppendingInteraction() must be called before appending interactions. And h%StartAppendingInteraction() must be called after all appending finished. These two subroutines should only (and must ) be called once. h%StartAppendingInteraction() can be called for many times in order to append all kinds of interactions.