
Introduction to MOLCAS
is a quantum chemistry software developed by scientists to be used by
scientists. It is not primarily a commercial product and it is not sold in
order to produce a fortune for its owner (the Lund University). The authors
have tried in MOLCAS to assemble their collected experience and knowledge in
computational quantum chemistry. MOLCAS is a research product and it is used
as a platform by the Lund quantum chemistry group in their work to develop new
and improved computational tools in quantum chemistry. Most of the codes in the
software have newly developed features and the user should not be surprised if
a bug is found now and then.
The basic philosophy behind MOLCAS is to develop methods that will allow an
accurate ab initio treatment of very general electronic structure problems
for molecular systems in both ground and excited states. This is not an easy
task. Our knowledge about how to obtain accurate properties for single
reference dominated ground states is today well developed and MOLCAS contains
a number of codes that can perform such calculations (MP2, CC, CPF, DFT etc).
All these methods treat the electron correlation starting from a single
determinant (closed or open shell) reference state. Such codes are today
standard in most quantum chemistry program systems.
However, the basic philosophy of MOLCAS is to be able to treat, at the same
level of accuracy also, highly degenerate states, such as those occurring in
excited states, at the transition state in some chemical reactions, in
diradicaloid systems, heavy metal systems, etc. This is a more difficult problem
since the single determinant approach will not work well in such cases. The key
feature of MOLCAS is the multiconfigurational approach. MOLCAS contains
codes for general and effective multiconfigurational SCF calculations at the
Complete Active Space (CASSCF) level, but also employing more restricted MCSCF
wave functions (RASSCF). It is also possible, at this level of theory, to
optimize geometries for equilibrium and transition states using gradient
techniques and to compute force fields and vibrational energies.
However, even if the RASSCF approach is known to give reasonable structures for
degenerate systems  both in ground and excited states  it is not in
general capable of recovering more than a small fraction of the correlation
energy. It is therefore necessary to supplement the multiconfigurational SCF
treatment with a calculation of the dynamic correlation effects. In the early
versions of MOLCAS, this was achieved by means of the multireference (MR) CI
method. This method has, however, severe limitations in the number of electrons
that can be correlated and the size of the reference space. It is not a method
that can be used to study excited states of anything but small molecules.
But here it has the capacity to produce very accurate wave functions and
potential surfaces. The MRCI code of MOLCAS is used by many groups for
this purpose. Today it is also possible to run the COLUMBUS MRCI code together
with MOLCAS.
In the years 198892, a new method was developed, which can be used to
compute dynamic electron correlation effects for multiconfigurational wave
functions. It is based on second order perturbation theory and has been
given the acronym CASPT2. It was included into the second version of
MOLCAS5. From the beginning it was not clear whether the CASPT2 method would be
accurate enough to be useful in practice. However, as it
turned out it was surprisingly accurate in a number of different types of
applications. The CASPT2 approach has become especially important in
studies of excited states and spectroscopic properties of large
molecules, where no other ab initio method has, so far, been applicable.
The method is based on second order perturbation theory and has therefore
limitations in accuracy, but the error limits have been investigated in a
large number of applications. The errors in relative energies are in
almost all cases small and the results can be used for conclusive
predictions about molecular properties in ground and excited states.
The major application areas for the CASPT2 method are potential energy
surfaces for chemical reactions and photochemistry. The method is under
constant development. Recently, a multistate version, which will allow the
simultaneous study of several electronic states, including their interaction in
second order. This code is especially useful in cases where two, or more energy
surfaces are close in energy. We have for a number of year also tried to develop
an analytical CASPT2 gradient code. For different reasons, this work is as yet
unfinished. Instead we have in the present version (7) included a
numerical procedure, which allows automatic geometry optimization at the CASPT2
level of theory. It is applicable to all states and systems for which the
CASPT2 energy can be computed.
The program RASSI has the
capacity to compute the interaction between several RASSCF wave functions based
on different orbitals, which are in general nonorthonormal (nonorthogonal CI).
RASSI is routinely used to compute transition dipole moments in spectroscopy,
but can also be used, for example, to study electron transfer or other
properties where it might be of value to use localized wave functions.
The size of the systems that can be treated with MOLCAS have been limited due
to limitations in storing twoelectron integrals for large basis set. This limit
has now been moved substantially to larger systems by the introduction of a
Cholesky decomposition of the twoelectron integrals. This feature is introduced
in MOLCAS7 and can be used for the SCF, CASSCF, CASPT2, RASSI and MP2 codes. It speeds up all calculations by orders
of magnitude and extends the size of the basis sets that can be used. The
accuracy can be controled by the threshold used in the decomposition. The same
approach can be used to generate R/I auxiliary basis sets on the fly, which can
then be used, for example to compute energy derivatives at the SCF, DFT; and
RASSCF levels of theory.
It should finally be clearly stated that MOLCAS is not a black box tool. The
user should be an educated quantum chemist, with some knowledge about the
different quantum chemical models in use today, their application areas and
their inherent accuracy. He should also have a critical mind and not take a
printed output for granted without checking that the results are in agreement
with his presumptions and consistent with the model he has employed. The skill
to use MOLCAS effectively does not come immediately, but we have tried to
help the user by providing together with this manual a book of examples, which
explains how some different key projects were solved using MOLCAS. We
are sure that the users will find them helpful in their own
attempts to master the software and use it in the chemical applications. The
MOLCAS group arranges regular MOLCAS workshops, which teaches how to use the
software.

