5.1 Toward More Complex Systems
163
the machine geometry. This direct comparison of theoretically predicted laboratory
distributions with experimental data avoids any arbitrariness of the models adopted
when analyzing of crossed beam data.
This procedure provides the most accurate evaluation of the quality of the adopted
ab initio PES and of the used dynamical treatment. It implies, however, the collaborative effort of research groups bearing different and complementary expertise.
Namely, expertise in
(a) Ab initio calculation of molecular electronic structure,
(b) Fitting of the potential energy values to a functional form,
(c) Running of molecular collision dynamical calculations,
(d) Assembling experimental observables from elementary properties
need to be properly combined. In particular, the forward convolution step performing
the appropriate averaging of detailed dynamical ab initio properties using a single
stream procedure to evaluate from first principles the N lab ((, t) distributions heavily
relies on a tight collaboration between computational chemists and experimentalists.
The measured N lab ((, t) (solid circles) are compared in Fig. 5.4 with the computed
(solid lines) lab PADs of CO 2 at E c = 14.1 Kcal mole
−1 for the Leiden (second panel
from the top), YMS (second panel from the bottom) and LTSH (bottom panel) PESs
obtained in this way [108].
Such computational procedure is, indeed, the one embodied into the so-called
grid empowered molecular simulator (GEMS) [107] that is a workflow articulated
into the following highly cooperative computational blocks:
• INTERACTION, for the generation and/or collection of single geometry ab initio
Born–Oppenheimer electronic structure values related to the potential energy surface of the system of interest for the whole range of relevant geometries. For this
purpose, one can choose among different suites of codes (including commercial
packages). This block can be skipped when use is made of precalculated ab initio
inputs.
• FITTING, for the use of either a global or a local interpolation of ab initio data
belonging to the same electronically adiabatic surface in terms of one or more
suitable functional forms (this block is skipped when an on the fly dynamical
approach is considered or a force field is adopted or a suitable grid of values of the
PES is already available from the literature). Most often the INTERACTION and
FITTING are chained together (including sometimes also the next block DYNAMICS) and iteratively used to the end of better shaping some critical regions of the
PES against information available from the literature.
• DYNAMICS, for the carrying out of dynamics calculations on the PES produced
by the FITTING block (or on the individual potential energy values provided by
ad hoc on the fly implemented ab initio packages). For few atom (at present only
atom–diatom) systems, the dynamics problem can be dealt with by using fulldimensional quantum mechanics techniques and converging with total angular
momentum. Approximate quantum, SC, QC, and QCT calculations can also be
used.
163
the machine geometry. This direct comparison of theoretically predicted laboratory
distributions with experimental data avoids any arbitrariness of the models adopted
when analyzing of crossed beam data.
This procedure provides the most accurate evaluation of the quality of the adopted
ab initio PES and of the used dynamical treatment. It implies, however, the collaborative effort of research groups bearing different and complementary expertise.
Namely, expertise in
(a) Ab initio calculation of molecular electronic structure,
(b) Fitting of the potential energy values to a functional form,
(c) Running of molecular collision dynamical calculations,
(d) Assembling experimental observables from elementary properties
need to be properly combined. In particular, the forward convolution step performing
the appropriate averaging of detailed dynamical ab initio properties using a single
stream procedure to evaluate from first principles the N lab ((, t) distributions heavily
relies on a tight collaboration between computational chemists and experimentalists.
The measured N lab ((, t) (solid circles) are compared in Fig. 5.4 with the computed
(solid lines) lab PADs of CO 2 at E c = 14.1 Kcal mole
−1 for the Leiden (second panel
from the top), YMS (second panel from the bottom) and LTSH (bottom panel) PESs
obtained in this way [108].
Such computational procedure is, indeed, the one embodied into the so-called
grid empowered molecular simulator (GEMS) [107] that is a workflow articulated
into the following highly cooperative computational blocks:
• INTERACTION, for the generation and/or collection of single geometry ab initio
Born–Oppenheimer electronic structure values related to the potential energy surface of the system of interest for the whole range of relevant geometries. For this
purpose, one can choose among different suites of codes (including commercial
packages). This block can be skipped when use is made of precalculated ab initio
inputs.
• FITTING, for the use of either a global or a local interpolation of ab initio data
belonging to the same electronically adiabatic surface in terms of one or more
suitable functional forms (this block is skipped when an on the fly dynamical
approach is considered or a force field is adopted or a suitable grid of values of the
PES is already available from the literature). Most often the INTERACTION and
FITTING are chained together (including sometimes also the next block DYNAMICS) and iteratively used to the end of better shaping some critical regions of the
PES against information available from the literature.
• DYNAMICS, for the carrying out of dynamics calculations on the PES produced
by the FITTING block (or on the individual potential energy values provided by
ad hoc on the fly implemented ab initio packages). For few atom (at present only
atom–diatom) systems, the dynamics problem can be dealt with by using fulldimensional quantum mechanics techniques and converging with total angular
momentum. Approximate quantum, SC, QC, and QCT calculations can also be
used.