! This example calculates the N 1s core-hole binding energy of pyridine
! using a delta-SCF procedure. It calculates a SCF solution in the
! ground state, then in the core-excited state, and reports the
! difference. The example shows how to obtain the core-excited state
! of a core of your choice.
geometry=pyridine.xyz

! we use default basis sets for all atoms except the one we wish
! to calculate the core hole of, in this case N 1s. For this atom,
! we uncontract the s and p functions of the basis to give the core
! hole more opportunities for relaxation.
!
! Notes:
!   - Uncontracting the complete basis would of course also work,
!     but might be more expensive
!   - For correlated calculations, you would need additional tight
!     correlation functions for the hole (e.g., use a cc-pwCVnZ basis
!     on the affected atom and cc-pVTZ on the rest)
basis={
   default,def2-TZVPP
   sp,N,def2-TZVPP     ! no "c;" following -- uncontracted.
   df,N,def2-TZVPP;c;  ! use d and f functions with contractions.
}

! reference calculation, normal DFT. We also uncontract the fitting
! basis to fit the core region more accurately.
{df-rks,pbe,df_basis=def2-tzvpp(u)}
ENORMAL = ENERGY       ! remember energy of ground state calculation.

! localize orbitals. This isolates the cores of the individual atoms
! also in the case of degeneracy. And, in particular, it shows us which
! orbital is the N 1s we are looking for. It comes out as orbital 1.1.
{ibba; save,2101.2}

! do the core-hole calculation. Additionally to the previous command, we
! specify nitord=1, which asks SCF to fix orbital order & occupations
! starting at iteration 1. This should prevent SCF from dropping down to
! the ground state SCF solution.
!
! With this, SCF will emit warnings about strongly deviating orbital
! occupations in the first iterations. This is to be expected when
! treating instable states and no reason for concern.
{df-rks,pbe,df_basis=def2-tzvpp(u),nitord=1;
   ! in the program, closed-shell orbitals always have lower numbers
   ! than open-shell orbitals. In order to get the core hole state, we
   ! thus need to exchange the localized N1s orbital (1.1) with whatever
   ! came out at the highest orbital number (21.1) before. [Because the
   ! latter one will be considered as singly-occupied. If we would not use
   ! localized orbitals, 21.1 would be the HOMO].
   rotate,1.1,21.1;    ! exchange orbitals 1.1 and 21.1
   orbital,2101.2;     ! use the localized orbitals as input
   wf,spin=1,charge=1  ! one open-shell orbital (implicitly 21.1)
}
EWITH_HOLE = ENERGY
! run population analysis again to see if we arrived at the right
! state. This should be indicated by an active orbital occupation
! of about 1.0 on the N 1s orbital.
{ibba}
! show the result, converted to electron volts.
{table,(EWITH_HOLE-ENORMAL)*toev
 title,N 1s core hole binding energy [Exp. value: (404.94 +/- 0.03) eV. [Can J. Chem 58 694 (1980)]]
}