146
3 Fundamentals of the Analysis Tools
Post-HF scheme:
This is the improved version of the HF method with the inclusion of an electroncorrelation effect and is expected to afford more quantitative result, especially in
the energy of a molecule. There are several entries in this scheme such as MP, CI,
CCSD(T), MCSCF, CASSCF, MRCI, and so on (see Sect. 3.1.2). The computation
time required becomes much longer than the ordinary non-empirical HF calculation
and much more memories for data storage becomes necessary at the same time.
This scheme affords comparatively quantitative results so that one should consider
the balance between the quality of the results and the computational resource when
employing this scheme.
DFT scheme:
Recent progress and improvement of the DFT method is remarkable and one can
obtain reasonably quantitative results out of this scheme. This has become rather
popular with recent experimental chemists for interpretation of their experimental
results as to the molecules and chemical reactions. The orbitals obtained are often
mentioned as also MO’s by the experimental users in spite of that strict theoreticians
rigorously tend to quote those as Kohn-Sham (KS) orbitals rather than MO’s.
There are several entries based on the hybrid functionals employed (B3LYP, B97,
PBED, and so on) and on the semiempirical functionals that can include long-range
corrections (LCBOP, CAM-B3LYP, ωB97X-D, and so on) (see Sect. 3.2). Among
these, for instance, it looks like that the current experimentalists mostly use the
B3LYP.
(2) Molecular mechanics (MM) method
This method is characterized by a molecular force field based on various kinds of
potential functions employed. There have been developed several kinds of molecular force field targeting for various chemical systems. For instance, TIP4P force
field is appropriate to the system including H 2 O molecules as solvent, AMBER
and CHARMM to biopolymer systems, and OPLS to liquid simulations. Moreover, MMFF94, MM2, MM3, MM4 force fields could be applicable to organic
molecules as well as biopolymers (also see Sect. 3.4). Hence, the force field ought
to be employed according to the chemical system one deals with.
Q2: What kind of basis set should be selected?
A2: It is crucial to employ an adequate basis set in the MO calculation. Basis set
implies the collective group of the basis functions to construct the LCAO used in the
MO calculation. In general, the more numbers of basis functions are included in a
basis set, the better results especially as to molecular energy will be provided. In other
words, such a basis set is referred to as that of higher quality but, as a matter of course,
it requires the longer computation time. There have been hitherto proposed quite a
few kinds of basis sets, which might be a bit ungraspable to ordinary users. Each
basis set, however, has its own structure, features, and systematic naming. Hence,
the users are encouraged to make out some of these and to select adequate one (ones)
3 Fundamentals of the Analysis Tools
Post-HF scheme:
This is the improved version of the HF method with the inclusion of an electroncorrelation effect and is expected to afford more quantitative result, especially in
the energy of a molecule. There are several entries in this scheme such as MP, CI,
CCSD(T), MCSCF, CASSCF, MRCI, and so on (see Sect. 3.1.2). The computation
time required becomes much longer than the ordinary non-empirical HF calculation
and much more memories for data storage becomes necessary at the same time.
This scheme affords comparatively quantitative results so that one should consider
the balance between the quality of the results and the computational resource when
employing this scheme.
DFT scheme:
Recent progress and improvement of the DFT method is remarkable and one can
obtain reasonably quantitative results out of this scheme. This has become rather
popular with recent experimental chemists for interpretation of their experimental
results as to the molecules and chemical reactions. The orbitals obtained are often
mentioned as also MO’s by the experimental users in spite of that strict theoreticians
rigorously tend to quote those as Kohn-Sham (KS) orbitals rather than MO’s.
There are several entries based on the hybrid functionals employed (B3LYP, B97,
PBED, and so on) and on the semiempirical functionals that can include long-range
corrections (LCBOP, CAM-B3LYP, ωB97X-D, and so on) (see Sect. 3.2). Among
these, for instance, it looks like that the current experimentalists mostly use the
B3LYP.
(2) Molecular mechanics (MM) method
This method is characterized by a molecular force field based on various kinds of
potential functions employed. There have been developed several kinds of molecular force field targeting for various chemical systems. For instance, TIP4P force
field is appropriate to the system including H 2 O molecules as solvent, AMBER
and CHARMM to biopolymer systems, and OPLS to liquid simulations. Moreover, MMFF94, MM2, MM3, MM4 force fields could be applicable to organic
molecules as well as biopolymers (also see Sect. 3.4). Hence, the force field ought
to be employed according to the chemical system one deals with.
Q2: What kind of basis set should be selected?
A2: It is crucial to employ an adequate basis set in the MO calculation. Basis set
implies the collective group of the basis functions to construct the LCAO used in the
MO calculation. In general, the more numbers of basis functions are included in a
basis set, the better results especially as to molecular energy will be provided. In other
words, such a basis set is referred to as that of higher quality but, as a matter of course,
it requires the longer computation time. There have been hitherto proposed quite a
few kinds of basis sets, which might be a bit ungraspable to ordinary users. Each
basis set, however, has its own structure, features, and systematic naming. Hence,
the users are encouraged to make out some of these and to select adequate one (ones)