diff --git a/src/mo_one_e_ints/orthonormalize.irp.f b/src/mo_one_e_ints/orthonormalize.irp.f
index cffc1993..aa8d85bc 100644
--- a/src/mo_one_e_ints/orthonormalize.irp.f
+++ b/src/mo_one_e_ints/orthonormalize.irp.f
@@ -1,11 +1,20 @@
 subroutine orthonormalize_mos
   implicit none
   integer :: m,p,s
-  m = size(mo_coef,1)
-  p = size(mo_overlap,1)
-  call ortho_lowdin(mo_overlap,p,mo_num,mo_coef,m,ao_num)
-  mo_label = 'Orthonormalized'
-  SOFT_TOUCH mo_coef mo_label
+  if (is_periodic) then
+    m = size(mo_coef_complex,1)
+    p = size(mo_overlap_complex,1)
+    call ortho_lowdin_complex(mo_overlap_complex,p,mo_num,mo_coef_complex,m,ao_num)
+    mo_label = 'Orthonormalized'
+    SOFT_TOUCH mo_coef_complex mo_label
+    !TODO: should we do anything with the separate real/imag parts of mo_coef_complex?
+  else
+    m = size(mo_coef,1)
+    p = size(mo_overlap,1)
+    call ortho_lowdin(mo_overlap,p,mo_num,mo_coef,m,ao_num)
+    mo_label = 'Orthonormalized'
+    SOFT_TOUCH mo_coef mo_label
+  endif
 end