Molcas features

Wave Functions, Energies, 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 functions.
• 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

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

• 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.

Environmental Effects

• 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.

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