![]() The DAVIDSON sub block may contain any number of records and must end with a record END. The simplest, fastest, and recommended way to obtain information about the ten lowest dipole-allowed excitation energies would be: ![]() The ALLOWED subkey automatically implies ONLYSING. Of course, the oscillator strengths may still be negligibly small. The subkey ALLOWED tells ADF to treat only those irreducible representations for which the oscillator strengths will be nonzero. This subkey should not be used in case of spin-orbit coupling. If you are interested in the optical absorption spectrum, you may not want to compute singlet-triplet excitation energies, nor singlet-singlet excitation energies which, by symmetry, have zero oscillator strengths. Dipole-allowed versus general excitations. One should in fact only use the results of the spin-polarized calculation. The subkeys ONLYSING and ONLYTRIP are misused in this case to do a spin-restricted calculation, or a spin-polarized calculation, respectively. In case of a calculation including spin-orbit coupling one can not separate the singlet-singlet and singlet-triplet excitations. ![]() One can skip one of these two parts of the calculation by specifying either ONLYSING or ONLYTRIP as a subkey in the data block. The singlets are handled first, then the corresponding triplet excitation energies. Singlet versus tripletīy default, the singlet-singlet and singlet-triplet excitation energies are both calculated. The EXACT option can not be used in unrestricted calculations. An advantage of the EXACT option is that additional information is produced, such as the Cauchy coefficients that determine the average dipole polarizability. A few of the lowest excitation energies and oscillator strengths are then found within an error tolerance. The default is the iterative Davidson method. It can be activated by specifying the block EXACT as one of the subkeys in the Excitations data block. ![]() Since the matrix may become very large, this option is possible only for very small molecules. The most straightforward procedure is a direct diagonalization of the matrix from which the excitation energies and oscillator strengths are obtained. Suitable defaults have been defined for all of these. In this case, the program needs to know how many excitation energies are needed per irrep, what accuracy is required, and what type of excitation energies are required (singlet-singlet or singlet-triplet). Two possible ways are available to solve the eigenvalue equation from which the excitation energies and oscillator strengths are obtained, of which the iterative Davidson procedure is the default. This functionality is based on TDDFT and consequently has a different theoretical foundation than the SCF techniques described elsewhere in this User’s Guide. Several options can be addressed with subkeys in the data block. EXCITATIONS EXACT END ALLOWED ONLYSING ONLYTRIP LOWEST nlowest End ![]()
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