bfield file not required when using only constraining fields

source/common/bfield.f90

@@ -252,8 +252,10 @@ contains
       if (ierror/=0) then
         write(*,*) '[read_bfield] bfield file does not exist'
         write(*,*) '              setting all bfields to zero'
-        write(*,*) '              disabling magnetic field lbfield = F'
-        bfield%lbfield        = .false.
+        if (.not.bfield%lbfield_constr) then
+          write(*,*) '              disabling magnetic field lbfield = F'
+          bfield%lbfield        = .false.
+        end if
         do iatom=1,natyp
           bfield%theta(iatom)            = 0.0D0
           bfield%phi(iatom)              = 0.0D0