Energy

The program prints the bonding energy (not in a Create or Frequencies run) and its decomposition in terms that are useful for chemical interpretation. The total energy is not computed. The bonding energy is defined relative to the fragments. When basic atoms are employed as fragments one should realize that these do not represent the atomic ground state since they are computed as spin-restricted and spherically symmetric objects, with possibly fractional occupation numbers. The correct multiplet state is not computed. To obtain the bonding energy with respect to isolated atoms you should therefore add atomic correction terms to account for spin polarization and the multiplet state. See also the SLATERDETERMINANTS key and the discussion in the Theory document on multiplet states.

The spin polarization energy can be computed by running the single atom unrestricted, using as fragment the corresponding (restricted) basic atom. The true multiplet state is not necessarily obtained in this way.

For the comparison of computed bonding energies with experimental data one should furthermore be aware of any aspects that are not represented in the computational formalism, such as zero-point motions and environment (solvent) effects.

In a Geometry Optimization or Transition State search, the program may print a bonding energy evaluation at each geometry (depending on print switches). A test-energy value is written in the log file. This is not the bonding energy, although the difference is usually small. The test-energy printed in the log file is the energy expression from which the energy gradients are computed. The true bonding energy contains in addition a few (small) correction terms that are mostly related to the fit incompleteness. These correction terms are usually very small.

If Electric Fields are used in the computation (homogeneous and/or point charges), the printed Bonding Energy is the energy of the molecule in the field minus the energy of the fragments in the same field. The energy terms due to the field are also printed separately so that one can subtract them from the total bonding energy to obtain the energy-change without field-terms.