5.1 Toward More Complex Systems
153
relativistic effects. It is used also by experimental chemists, physicists, and biologist
interested in rationalizing their measurements thanks to its user-friendly style.
HONDOPLUS [82] a modified version of the HONDO-v99.6 electronic structure
program that includes solvation methods and other capabilities among which diabatization (by defining diabatic molecular orbitals (DMOs) to reformulate CASSCF
or MC-QDPT wave functions) and new methods to calculate partial atomic charges,
avoid intruder states, extend the basis sets, introduce user-defined density functionals,
and improve portability (see http://t1.chem.umn.edu/hondoplus/).
CADPAC [83] a suite of programs for ab initio computational chemistry calculations. The package can perform SCF (RHF, ROHF, UHF and GRHF, analytic
gradients and force constants (numerical for UHF), location of stationary points
for all SCF types) Møller–Plesset and other correlated electronic structure (multipole moments, distributed multipole analysis, polarizabilities, magnetizabilities,
NMR shielding constants, infrared intensities, Raman intensities, VCD intensities,
frequency-dependent polarizabilities and hyperpolarizabilities, excitation energies
by RPA method, dispersion coefficients, effects of external fields, field gradients,
and lattices) calculations (see http://en.wikipedia.org/wiki/CADPAC).
All these packages (and several other more popular in specific research areas)
are designed for computing ab initio potential energy values following a Born–
Oppenheimer approximation. Most of them (either open source or commercial),
however, are articulated into several separate programs and have been often developed
over many years as a stratification of heterogenous pieces of software. This makes
it difficult to further develop and restructure them for efficiently running on modern
computer architectures. Moreover, most of them are mainly designed to calculate
the electronic structure of molecular systems around equilibrium. This gives rise
to discontinuity problems (like those associated with the convergence to the right
electronic state when dealing with dynamics problems in which the partners are
driven to explore large intramolecular distances).
5.1.2 Fitting PESs for Reactive and Nonreactive Channels
The problem of accurately determining reactive and nonreactive channels of a PES
goes together with that of properly fitting a (large) set of calculated ab initio electronic
structure values. Traditionally, this has involved, in particular, the strong interaction
regions in which the exchange of energy and matter between atoms and molecules
occurs. This has been discussed already in Chap. 3 where, in the spirit of the MPE
(many-process expansion) approach, specific attention was paid to the processes in
which the structured nature of the collision partners is responsible for an intriguing
pattern of energy exchanges (including the distortion of molecular arrangements,
their dissociation and reassociation, etc.) as well as long-range orientations and vector
correlations regardless of whether the exchange of mass (reaction) occurs or not.
In the reaction-oriented approach of the present book, strong emphasis is given to
the role played by the long-range part of the interaction in approaching the strong
153
relativistic effects. It is used also by experimental chemists, physicists, and biologist
interested in rationalizing their measurements thanks to its user-friendly style.
HONDOPLUS [82] a modified version of the HONDO-v99.6 electronic structure
program that includes solvation methods and other capabilities among which diabatization (by defining diabatic molecular orbitals (DMOs) to reformulate CASSCF
or MC-QDPT wave functions) and new methods to calculate partial atomic charges,
avoid intruder states, extend the basis sets, introduce user-defined density functionals,
and improve portability (see http://t1.chem.umn.edu/hondoplus/).
CADPAC [83] a suite of programs for ab initio computational chemistry calculations. The package can perform SCF (RHF, ROHF, UHF and GRHF, analytic
gradients and force constants (numerical for UHF), location of stationary points
for all SCF types) Møller–Plesset and other correlated electronic structure (multipole moments, distributed multipole analysis, polarizabilities, magnetizabilities,
NMR shielding constants, infrared intensities, Raman intensities, VCD intensities,
frequency-dependent polarizabilities and hyperpolarizabilities, excitation energies
by RPA method, dispersion coefficients, effects of external fields, field gradients,
and lattices) calculations (see http://en.wikipedia.org/wiki/CADPAC).
All these packages (and several other more popular in specific research areas)
are designed for computing ab initio potential energy values following a Born–
Oppenheimer approximation. Most of them (either open source or commercial),
however, are articulated into several separate programs and have been often developed
over many years as a stratification of heterogenous pieces of software. This makes
it difficult to further develop and restructure them for efficiently running on modern
computer architectures. Moreover, most of them are mainly designed to calculate
the electronic structure of molecular systems around equilibrium. This gives rise
to discontinuity problems (like those associated with the convergence to the right
electronic state when dealing with dynamics problems in which the partners are
driven to explore large intramolecular distances).
5.1.2 Fitting PESs for Reactive and Nonreactive Channels
The problem of accurately determining reactive and nonreactive channels of a PES
goes together with that of properly fitting a (large) set of calculated ab initio electronic
structure values. Traditionally, this has involved, in particular, the strong interaction
regions in which the exchange of energy and matter between atoms and molecules
occurs. This has been discussed already in Chap. 3 where, in the spirit of the MPE
(many-process expansion) approach, specific attention was paid to the processes in
which the structured nature of the collision partners is responsible for an intriguing
pattern of energy exchanges (including the distortion of molecular arrangements,
their dissociation and reassociation, etc.) as well as long-range orientations and vector
correlations regardless of whether the exchange of mass (reaction) occurs or not.
In the reaction-oriented approach of the present book, strong emphasis is given to
the role played by the long-range part of the interaction in approaching the strong