diff --git a/src/mo_guess/h_core_guess_routine.irp.f b/src/mo_guess/h_core_guess_routine.irp.f
index 8fc3f6f2..429f77ec 100644
--- a/src/mo_guess/h_core_guess_routine.irp.f
+++ b/src/mo_guess/h_core_guess_routine.irp.f
@@ -5,9 +5,19 @@ subroutine hcore_guess
   implicit none
   character*(64)                 :: label
   label = "Guess"
-  call mo_as_eigvectors_of_mo_matrix(mo_one_e_integrals,          &
-                                     size(mo_one_e_integrals,1),  &
-                                     size(mo_one_e_integrals,2),label,1,.false.)
-  call save_mos
-  SOFT_TOUCH mo_coef mo_label
+  if (is_periodic) then
+    call mo_as_eigvectors_of_mo_matrix_complex(mo_one_e_integrals_complex,  &
+                                       size(mo_one_e_integrals_complex,1),  &
+                                       size(mo_one_e_integrals_complex,2),label,1,.false.)
+    call save_mos
+    !TODO: is this correct? decide how to handle separate real/imag parts of mo_coef
+    SOFT_TOUCH mo_coef_complex mo_label
+
+  else
+    call mo_as_eigvectors_of_mo_matrix(mo_one_e_integrals,          &
+                                       size(mo_one_e_integrals,1),  &
+                                       size(mo_one_e_integrals,2),label,1,.false.)
+    call save_mos
+    SOFT_TOUCH mo_coef mo_label
+  endif
 end
diff --git a/src/mo_guess/mo_ortho_lowdin_complex.irp.f b/src/mo_guess/mo_ortho_lowdin_complex.irp.f
new file mode 100644
index 00000000..5e1dacbe
--- /dev/null
+++ b/src/mo_guess/mo_ortho_lowdin_complex.irp.f
@@ -0,0 +1,48 @@
+BEGIN_PROVIDER [complex*16, ao_ortho_lowdin_coef_complex, (ao_num,ao_num)]
+  implicit none
+  BEGIN_DOC
+! matrix of the coefficients of the mos generated by the
+! orthonormalization by the S^{-1/2} canonical transformation of the aos
+! ao_ortho_lowdin_coef(i,j) = coefficient of the ith ao on the jth ao_ortho_lowdin orbital
+  END_DOC
+  integer                        :: i,j,k,l
+  complex*16, allocatable  :: tmp_matrix(:,:)
+  allocate (tmp_matrix(ao_num,ao_num))
+  tmp_matrix(:,:) = (0.d0,0.d0)
+  do j=1, ao_num
+    tmp_matrix(j,j) = (1.d0,0.d0)
+  enddo
+  call ortho_lowdin_complex(ao_overlap_complex,ao_num,ao_num,tmp_matrix,ao_num,ao_num)
+  do i=1, ao_num
+    do j=1, ao_num
+      ao_ortho_lowdin_coef_complex(j,i) = tmp_matrix(i,j)
+    enddo
+  enddo
+  deallocate(tmp_matrix)
+END_PROVIDER
+
+BEGIN_PROVIDER [complex*16, ao_ortho_lowdin_overlap_complex, (ao_num,ao_num)]
+  implicit none
+  BEGIN_DOC
+! overlap matrix of the ao_ortho_lowdin
+! supposed to be the Identity
+  END_DOC
+  integer                        :: i,j,k,l
+  complex*16               :: c
+  do j=1, ao_num
+    do i=1, ao_num
+      ao_ortho_lowdin_overlap_complex(i,j) = (0.d0,0.d0)
+    enddo
+  enddo
+  do k=1, ao_num
+    do j=1, ao_num
+      c = (0.d0,0.d0)
+      do l=1, ao_num
+        c +=  dconjg(ao_ortho_lowdin_coef_complex(j,l)) * ao_overlap_complex(k,l)
+      enddo
+      do i=1, ao_num
+        ao_ortho_lowdin_overlap_complex(i,j) += ao_ortho_lowdin_coef_complex(i,k) * c
+      enddo
+    enddo
+  enddo
+END_PROVIDER
diff --git a/src/mo_guess/pot_mo_ortho_canonical_ints.irp.f b/src/mo_guess/pot_mo_ortho_canonical_ints.irp.f
index 984d45a5..10363a00 100644
--- a/src/mo_guess/pot_mo_ortho_canonical_ints.irp.f
+++ b/src/mo_guess/pot_mo_ortho_canonical_ints.irp.f
@@ -23,3 +23,29 @@ BEGIN_PROVIDER [double precision, ao_ortho_canonical_nucl_elec_integrals, (mo_nu
  !$OMP END PARALLEL DO
 END_PROVIDER
 
+BEGIN_PROVIDER [complex*16, ao_ortho_canonical_nucl_elec_integrals_complex, (mo_num,mo_num)]
+ implicit none
+ integer :: i1,j1,i,j
+ complex*16 :: c_i1,c_j1
+
+ ao_ortho_canonical_nucl_elec_integrals_complex = (0.d0,0.d0)
+ !$OMP PARALLEL DO DEFAULT(none) &
+ !$OMP PRIVATE(i,j,i1,j1,c_j1,c_i1) &
+ !$OMP SHARED(mo_num,ao_num,ao_ortho_canonical_coef_complex, &
+ !$OMP   ao_ortho_canonical_nucl_elec_integrals_complex, ao_integrals_n_e_complex)
+ do i = 1, mo_num
+   do j = 1, mo_num
+    do i1 = 1,ao_num
+     c_i1 = ao_ortho_canonical_coef_complex(i1,i)
+     do j1 = 1,ao_num
+       c_j1 = c_i1*dconjg(ao_ortho_canonical_coef_complex(j1,j))
+       ao_ortho_canonical_nucl_elec_integrals_complex(j,i) = &
+               ao_ortho_canonical_nucl_elec_integrals_complex(j,i) + &
+                c_j1 * ao_integrals_n_e_complex(j1,i1)
+     enddo
+    enddo
+   enddo
+ enddo
+ !$OMP END PARALLEL DO
+END_PROVIDER
+
diff --git a/src/mo_guess/pot_mo_ortho_lowdin_ints.irp.f b/src/mo_guess/pot_mo_ortho_lowdin_ints.irp.f
index 5a9f9978..7ec94296 100644
--- a/src/mo_guess/pot_mo_ortho_lowdin_ints.irp.f
+++ b/src/mo_guess/pot_mo_ortho_lowdin_ints.irp.f
@@ -23,3 +23,29 @@ BEGIN_PROVIDER [double precision, ao_ortho_lowdin_nucl_elec_integrals, (mo_num,m
  !$OMP END PARALLEL DO
 END_PROVIDER
 
+BEGIN_PROVIDER [complex*16, ao_ortho_lowdin_nucl_elec_integrals_complex, (mo_num,mo_num)]
+ implicit none
+ integer :: i1,j1,i,j
+ complex*16 :: c_i1,c_j1
+
+ ao_ortho_lowdin_nucl_elec_integrals = (0.d0,0.d0)
+ !$OMP PARALLEL DO DEFAULT(none) &
+ !$OMP PRIVATE(i,j,i1,j1,c_j1,c_i1) &
+ !$OMP SHARED(mo_num,ao_num,ao_ortho_lowdin_coef_complex, &
+ !$OMP   ao_ortho_lowdin_nucl_elec_integrals_complex, ao_integrals_n_e_complex)
+ do i = 1, mo_num
+   do j = 1, mo_num
+    do i1 = 1,ao_num
+     c_i1 = ao_ortho_lowdin_coef_complex(i1,i)
+     do j1 = 1,ao_num
+       c_j1 = c_i1*dconjg(ao_ortho_lowdin_coef_complex(j1,j))
+       ao_ortho_lowdin_nucl_elec_integrals_complex(j,i) = &
+         ao_ortho_lowdin_nucl_elec_integrals_complex(j,i) + &
+                                    c_j1 * ao_integrals_n_e_complex(j1,i1)
+     enddo
+    enddo
+   enddo
+ enddo
+ !$OMP END PARALLEL DO
+END_PROVIDER
+