3.6 Hints for Calculations
145
out of the current calculation resources for their own purpose to obtain the effective
results to reveal, support, or augment the points they need to know in the course of
the experimental research.
There can be miscellaneous chemical subjects for the users to search for as a matter
of course. These demands would be related to, for instance, structure, electronic
properties, chemical reactivities, and so on with respect to molecules and polymers.
In this section, we attempt to afford certain hints and/or prescriptions using the Q &
A style toward the effective selection of the calculation method from practical points
of view.
Q1: What kind of theoretical calculations can be used for molecules?
A1: There are several categories of calculation methods as summarized below.
Of these should be employed appropriate one (ones) considering motivation and
demand in one’s research, since there are certain characteristics and differences in
these methods including reasonable cost performances such as computation time and
memory size.
(1) Molecular orbital (MO) method
This is to analyze the electronic structure of molecules in terms of the MO. The
MO can also be extended to 1D polymer with the introduction of special boundary
conditions to the MO’s to the CO’s. A detailed calculation scheme in the MO or the
CO method can be roughly classified into the following four:
Empirical scheme:
This scheme includes the Hückel and extended Hückel methods. Common features
of these are that the computation time is extremely short and the output is rather
qualitative. The former method is able to deal with only π electrons and the latter
both σ and π electrons. It is noted that these methods cannot discuss the energy of
molecules due to their theoretically simple frameworks. However, the MO patterns
obtained are sometimes useful for a qualitative discussion of chemical reaction of
the concerning molecule(s).
HF scheme:
This can be classified into the semiempirical and the non-empirical (also used to
be called ab initio) HF methods (see Sect. 3.1.1). The semiempirical method is
equipped with several entries such as CNDO, INDO, MINDO, CNDO/S, ZINDO,
MNDO, AM1, PM6, and others, all of which deal with only the valence electrons
requiring quite short computation time and giving qualitative results. The semiempirical method is sometimes convenient for certain purposes to well reproduce the
excitation energies, for instance, due to the employment of the parameters fitted to
the experimental data.
On the other hand, the non-empirical method is characterized by that it does not
contain any experimental parameters, while it requires rather longer computation
time and gives semiquantitative result largely depending on the basis sets employed.
145
out of the current calculation resources for their own purpose to obtain the effective
results to reveal, support, or augment the points they need to know in the course of
the experimental research.
There can be miscellaneous chemical subjects for the users to search for as a matter
of course. These demands would be related to, for instance, structure, electronic
properties, chemical reactivities, and so on with respect to molecules and polymers.
In this section, we attempt to afford certain hints and/or prescriptions using the Q &
A style toward the effective selection of the calculation method from practical points
of view.
Q1: What kind of theoretical calculations can be used for molecules?
A1: There are several categories of calculation methods as summarized below.
Of these should be employed appropriate one (ones) considering motivation and
demand in one’s research, since there are certain characteristics and differences in
these methods including reasonable cost performances such as computation time and
memory size.
(1) Molecular orbital (MO) method
This is to analyze the electronic structure of molecules in terms of the MO. The
MO can also be extended to 1D polymer with the introduction of special boundary
conditions to the MO’s to the CO’s. A detailed calculation scheme in the MO or the
CO method can be roughly classified into the following four:
Empirical scheme:
This scheme includes the Hückel and extended Hückel methods. Common features
of these are that the computation time is extremely short and the output is rather
qualitative. The former method is able to deal with only π electrons and the latter
both σ and π electrons. It is noted that these methods cannot discuss the energy of
molecules due to their theoretically simple frameworks. However, the MO patterns
obtained are sometimes useful for a qualitative discussion of chemical reaction of
the concerning molecule(s).
HF scheme:
This can be classified into the semiempirical and the non-empirical (also used to
be called ab initio) HF methods (see Sect. 3.1.1). The semiempirical method is
equipped with several entries such as CNDO, INDO, MINDO, CNDO/S, ZINDO,
MNDO, AM1, PM6, and others, all of which deal with only the valence electrons
requiring quite short computation time and giving qualitative results. The semiempirical method is sometimes convenient for certain purposes to well reproduce the
excitation energies, for instance, due to the employment of the parameters fitted to
the experimental data.
On the other hand, the non-empirical method is characterized by that it does not
contain any experimental parameters, while it requires rather longer computation
time and gives semiquantitative result largely depending on the basis sets employed.