add create_excitations.irp.f

src/Dets/create_excitations.irp.f

@@ -0,0 +1,36 @@
+subroutine do_mono_excitation(key_in,i_hole,i_particle,ispin,i_ok)
+ implicit none
+ BEGIN_DOC
+ ! Apply the mono excitation operator : a^{dager}_(i_particle) a_(i_hole) of spin = ispin
+ ! on key_in 
+ ! ispin = 1  == alpha
+ ! ispin = 2  == beta
+ ! i_ok = 1  == the excitation is possible
+ ! i_ok = -1 == the excitation is not possible
+ END_DOC
+ integer, intent(in) :: i_hole,i_particle,ispin
+ integer(bit_kind), intent(inout) :: key_in(N_int,2)
+ integer, intent(out) :: i_ok
+ integer :: k,j,i
+ use bitmasks
+ ASSERT (i_hole > 0 )
+ ASSERT (i_particle <= mo_tot_num)
+ i_ok = 1
+ ! hole
+ k = ishft(i_hole-1,-bit_kind_shift)+1
+ j = i_hole-ishft(k-1,bit_kind_shift)-1
+ key_in(k,ispin) = ibclr(key_in(k,ispin),j)
+
+ ! particle
+ k = ishft(i_particle-1,-bit_kind_shift)+1
+ j = i_particle-ishft(k-1,bit_kind_shift)-1
+ key_in(k,ispin) = ibset(key_in(k,ispin),j)
+ integer :: n_elec_tmp
+ n_elec_tmp = 0    
+ do i = 1, N_int
+  n_elec_tmp += popcnt(key_in(i,1)) + popcnt(key_in(i,2))
+ enddo
+ if(n_elec_tmp .ne. elec_num)then
+  i_ok = -1
+ endif
+end

tests/methane.xyz

@@ -0,0 +1,7 @@
+5
+methane molecule (in ångströms)
+C        0.000000        0.000000        0.000000
+H        0.000000        0.000000        1.089000
+H        1.026719        0.000000       -0.363000
+H       -0.513360       -0.889165       -0.363000
+H       -0.513360        0.889165       -0.363000