Question

Which of the following statements about semi-empirical methods are true: It can be used for static molecules but not for reactions in which bonds are forming or breaking. It combines a theoretically sound model of bonding with some experimentally determined parameters. It can determine precisely both the location and the energy of bonding electrons. It is good at predicting structural properties that arise from subtle interactions of orbitals.

Answer 1

Answer:

The true statement about semi-empirical methods is:

It combines a theoretically sound model of bonding with some experimentally determined parameters.

Explanation:

Semi-empirical methods are theoretical assumptions combined with experimental data to improve computational performance.  In modern computational chemistry, semi-empirical methods play important roles. The methods offer the advantage of a substantial reduction in computational time.  They also increase one's ability to carry out calculations for large molecules.  However, with these methods, there is the added loss of the variational principle because one can obtain a total energy that is below the true total energy.  Again, semi-empirical methods have limited applicability because the methods apply to molecules with parameterized elements.  Semi-empirical methods may supply spurious results, especially since the molecular systems may be different from those used for parameterizations.