As a developer of MOKIT, I would like to recommend the automatic CASSCF computation using the
automr utility in MOKIT. We do not need to manually choose/permute orbitals, no need to specify the active space size, no need to start from a minimal basis set, and no need to take care about the input format of Molpro. All these things can be determined automatically.
Firstly, we write a .gjf file
C -0.39943387 0.82617525 0.16329935
H -0.04276103 1.33057344 -0.71035216
H -1.46943387 0.82618843 0.16329935
Cl 0.36724607 1.91039566 2.04124183
This is just Gaussian gjf format and it can be visualized, e.g. using GaussView.
CASSCF_prog=Molpro means you want Molpro to do the CASSCF computation.
Secondly, submit the job
automr CH2Cl.gjf >CH2Cl.out 2>&1
It takes less than 3 minutes and CASSCF is converged in 2 cycles.
automr will call Gaussian, GAMESS, Molpro programs successively (in addtion, PySCF is needed), this procedure is actually UHF->UNO->localized paired UNO->GVB->CASSCF. You can find the CASSCF active space size is determined as (3,3) in the output file
CH2Cl.out. Related energies and unpaired electrons are printed
E(CASCI) = -498.47159238 a.u.
E(CASSCF) = -498.47282188 a.u.
----------------------- Radical index -----------------------
Not spin singlet. Biradical character will not be computed.
Yamaguchi's unpaired electrons (sum_n n(2-n) ): 1.438
Head-Gordon's unpaired electrons(sum_n min(n,(2-n))): 1.230
Head-Gordon's unpaired electrons(sum_n (n(2-n))^2 ): 1.096
The CASSCF initial orbitals and converged orbitals are stored in files
respectively, which can be visualized using GaussView, Multiwfn, etc. The CAS(3,3) contains the singly occupied orbital and C-Cl bonding as well as anti-bonding orbitals:
(natural orbital occupation numbers are shown below the isosurfaces)
Thirdly, in this step the user has two choices. The first choice is reading CASSCF orbitals of the current geometry and performing Molpro CASSCF calculations of other geometries. In the step above we will obtain files
The converged CASSCF orbitals are stored in the
.xml file. And you can modify the file
CH2Cl_uhf_gvb6_CASSCF.com to change geometries.
automr is not involved here.
The second choice is again to use
automr to perform automatic calculations for each geometry. In this case you may need to specify
CASSCF(3,3) in the .gjf file when C-Cl bond is near its equilibrium bond length. This is because a not-stretched C-Cl bond has no multiconfigurational character and thus will not be put into the active space. If you do not specify
CASSCF(3,3) in such case,
automr may recommend
Of course, you may think CAS(3,3) is a minimal active space and you want a larger one. This can be easily achieved since all bonding and anti-bonding orbitals are stores in
CH2Cl_uhf_uno_asrot2gvb6_s.fch, in which you will also find two C-H bonds, C,Cl core orbitals and Cl valence orbitals. All these orbitals are spatially localized and can be identified with no effort. You can simply perform a CASSCF(13e,10o) calculation using Molpro without permuting orbitals, here the active space includes also the bonding and anti-bonding orbitals of two C-H bonds, three Cl 3s/3p orbitals.