Wave Functions, Energies, Properties
MOLCAS uses the following wave function models in calculations of total
energies, electronic structures and molecular properties:
- Hartree-Fock combined with DFT. Direct or semi-direct schemes are used, which
makes possible calculations at this level with more than a thousand basis
- Møller-Plesset second order perturbation theory (closed or restricted open
shell) can be used for a cheap estimate of dynamic electron correlation effects.
- Multiconfigurational SCF (CAS or RAS) is used to treat systems where the
electronic structure is not well described by a single determinant. Allows state
averaging for treatment of many electronic states.
Wave function up to about a million electronic configurations can be studied.
- Multiconfigurational second order perturbation theory (CASPT2) can be used to
estimate dynamic electron correlation for electronic states obtained with the
CASSCF method. A multi-state version allows the reference states to be modified
by the correlation using an effective Hamiltonian approach.
- For small molecules the multi-reference CI (MRCI) method can yield highly
accurate wave functions and energies.
- Molecules and radicals, well described by a single determinant, can
also be studied using coupled-cluster theory
(closed shell and restricted open shell CCSD(T)).
Molecular Structures, Vibrational Frequencies, Thermodynamics
Automatic geometry optimization using analytical or numerical gradient
techniques are available. These procedure can be used to obtain
equilibrium geometries, transition states, etc. both for ground and
excited electronic states.
Vibrational frequencies and thermodynamical quantities are computed for
SCF/CASSCF wave functions using analytical second derivatives. Numerical
frequencies are also available.
The resolution of identity approach (RI) and Cholesky decomposition (CD)
is available at the HF/DFT/MP2/RASSCF/CASPT2 level of approximation.
Analytic gradients are implemented for "pure" DFT. Analytic gradients to
follow for HF, hybrid DFT and RASSCF in the near furture.
Excited States and Electronic Spectra
MOLCAS is in particular designed to study potential surfaces for excited states.
- Energies may be obtained using all the wave function methods. Geometry
optimization is possible also for state average RASSCF energies.
- Transition properties are computed at the RASSCF level using the RASSCF State
Interaction Method, which is unique to the MOLCAS program.
- The same code can also be used to compute spin-orbit coupling using a effective
one-electron SO Hamiltonian and so called Atomic Mean Field Integrals (AMFI).
- Automatic search at the RASSCF level for energy barriers, conical
intersections, etc on excited state surfaces.
- Vibrationally resolved electronic spectra may be obtained using the MULA code
for computing transition dipole moments between harmonic vibrational levels of
two electronic states.
MOLCAS gives new possibilities to treat molecules in solutions
and macromolecular systems.
- Solvent effects can be treated using the Onsager spherical cavity model or
the Polarizable Continuum Model (PCM).
- Combined QM and molecular Mechanics (MM) calculations can be performed on
macromolecular systems like proteins, molecular clusters (droplets), etc.
There is an online interface to run Molcas on one of our servers, we call
this interface the Molcas farm. You can create an input, submit it and
retrieve the result. Usage is limited to 5 minutes of CPU time.
Mail to email@example.com to get an access to Molcas farm.