B..10 Facilities that were new in MOLPRO98

MOLPRO98 has the full functionality of MOLPRO96, but in order
to make the code more modular and easier to use and maintain, a number of structural changes
have been made. In particular, the number of different records has been significantly reduced.
The information for a given wavefunction type, like orbitals, density matrices, fock matrices,
occupation numbers and other information, is now stored in a
single dump record. Even different orbital types, e.g., canonical, natural, or localized
orbitals, are stored in the same record, and the user can subsequently access individual sets
by keywords on the `ORBITAL` directive. New facilities allow the use of starting
orbitals computed with different basis sets and/or different symmetries for SCF or MCSCF
calculations. The default starting guess for SCF calculations has been much improved, which is most
useful in calculations for large molecules. The use of special procedures for computing
non-adiabatic couplings or diabatization of orbitals has been significantly simplified. We hope that
these changes make the program easier to use and reduce the probability of input errors.
However, in order to use the new facilities efficiently, even experienced MOLPRO users should
read the sections
*RECORDS* and
*SELECTING ORBITALS AND DENSITY MATRICES* in the manual.
It is likely that standard MOLPRO96 inputs still work, but
changes may be required in more special cases involving particular records for orbitals,
density matrices, or operators.

All one-electron operators needed to compute expectation values and transition
quantities are now stored in a single record. Operators for which expectation values are requested
can be selected globally for all programs of a given run using the global `GEXPEC` directive,
or for a specific program using the `EXPEC` directive. All operators are computed
automatically when needed, and the user does not have to give input for this any more.
See section
*ONE-ELECTRON OPERATORES AND EXPECTATION VALUES* of the manual for details.

Due to the changed structure of dump and operator records, the utility program `MATROP`
has a new input syntax. MOLPRO96 inputs for `MATROP` do not work any more.

In addition to these organizational changes, a number of new programs have been added. Analytic energy gradients can now be evaluated for MP2 and DFT wavefunctions, and harmonic vibrational frequencies, intensities, and thermodynamic quantities can be computed automatically using finite differences of analytical gradients. Geometry optimization has been further improved, and new facilities for reaction path following have been added.

An interface to the graphics program MOLDEN has been added, which allows to visualize molecular structures, orbitals, electron densities, or vibrations.

Integral-direct calculations, in which the two-electron integrals in the AO basis are never
stored on disk but always recomputed when needed, are now available for all kinds of
wavefunctions, with the exception of perturbative triple excitations in MP4 and CCSD(T)
calculations. This allows the use of significantly larger basis sets than was possible before.
The direct option can be selected globally using the `GDIRECT` command, or for a
specific program using the `DIRECT` directive. See section
*INTEGRAL DIRECT METHODS* in the manual
for details. Note that the `DIRECT` module is optional and not part of the basic
MOLPRO distribution.

*Local* electron correlation methods have been further improved. In combination with
the integral-direct modules, which implement efficient prescreening techniques, the scaling
of the computational cost with molecular size is dramatically reduced, approaching now
quadratic or even linear scaling for MP2 and higher correlation methods. This makes possible
to perform correlated calculations for much larger molecules than were previously feasible.
However, since these methods are subject of active current research and still under intense
development, we decided not to include them in the current MOLPRO release. They
will be optionally available in one of the next releases.