Input File¶
Complete Input¶
! def2-SVP
!moread
%moinp "dae-rhf.gbw"
%pal nprocs 16 end
%casscf
nel 4
norb 4
nroots 3
mult 1
end
*xyzfile 0 1 dae.xyz
Line-by-Line¶
| Line | Meaning |
|---|---|
! def2-SVP |
Basis set for the calculation. |
!moread |
Tells ORCA to read in a starting set of molecular orbitals rather than generating an initial guess itself. |
%moinp "dae-rhf.gbw" |
The specific orbital file to read — the converged RHF orbitals from the RHF tutorial. |
%pal nprocs 16 end |
Runs the calculation in parallel across 16 cores. |
%casscf ... end |
The CASSCF block, detailed below. |
*xyzfile 0 1 dae.xyz |
Charge 0, multiplicity 1 (closed-shell singlet), geometry read from dae.xyz. |
The %casscf Block¶
| Keyword | Value | Meaning |
|---|---|---|
nel |
4 | Number of active electrons. |
norb |
4 | Number of active orbitals. |
nroots |
3 | Number of electronic states to compute (state-averaged). |
mult |
1 | Spin multiplicity of the states (singlet). |
This defines a CAS(4,4) calculation over 3 singlet states — see The Active Space Concept for what choosing an active space actually means, and State-Averaged CASSCF for what computing multiple nroots at once implies for the orbital optimization.
Where the Geometry Came From¶
dae.xyz was not built by hand — it was converted directly from an already-optimized Gaussian geometry using Open Babel:
This is the same conversion pattern used in the Electronic Structure Theory workflow, just with an xyz output format instead of a GAMESS input — Open Babel supports both from the same Gaussian log file.
Where the RHF Orbitals Came From¶
dae-rhf.gbw is not generated by this input — it's the converged orbital file produced by the separate RHF calculation covered in the RHF tutorial, read in here via %moinp. Reading in RHF orbitals rather than letting ORCA build its own initial guess gives the CASSCF orbital optimization a physically reasonable starting point, and is the same reason GUESS=MOREAD is used going from RHF into CIS in the GAMESS CIS tutorial.