visualization exercises are described in publications by Wavefunction, Inc.,
e.g. [ 328 ].
5.6 Strengths and Weaknesses of Ab initio Calculations...................
5.6.1 Strengths..........................................................
Ab initio calculations are based on a fundamental physical equation, the Schr€odinger
equation, without empirical adjustments. This makes them esthetically satisfying,
and ensures (if the Schr€odinger equation is true) that they will give correct answers
provided the approximations needed to obtain numerical results (to solve the
Schrodinger equation) are not too severe for the problem at hand€. The level of
theory needed for a reliable answer to a particular problem must be found by
experience – comparison with experiment for related cases – so in this sense current
ab initio calculations are not fully a priori [ 2 , 43 ]. A few “ab initio methods” do not
even fully eschew empirical parameters: the Gaussian and the CBS series of
methods have empirical factors which, unless they cancel (as in proton affinity
calculations, Section 5.2.2.2b) make these methods, strictly speaking, semiempiri-
cal. A consequence of the (usual) absence of empirical parameters is that ab initio
calculations can be performed for any kind of molecular species, including transi-
tion states and even non-stationary points, rather than only species for which
empirical parameters are available (seeChapter 6). These characteristics of reli-
ability (with the caveats alluded to) and generality are the strengths of ab initio
calculations.
5.6.2 Weaknesses.......................................................
Compared to other methods (molecular mechanics, semiempirical calculations,
density functional calculations – Chapters 3 , 6 and 7 , respectively) ab initio
calculations are slow, and they are relatively demanding of computer resources
(memory and disk space, depending on the program and the particular calculation).
These disadvantages, which increase with the level of the calculation, have been to
a very large extent overcome by the tremendous increase in computer power,
accompanied by decreases in price, that have taken place since the invention of
electronic computers. In 1959 Coulson doubted the possibility (he also questioned
the desirability, but in this regard visualization has been of enormous help) of
calculations on molecules with more than 20 electrons, but 30 years later computer
speed had increased by a factor of 100,000 [ 329 ], and ab initio calculations on
molecules with 100 electrons (about 15 heavy atoms) are common.
372 5 Ab initio Calculations