$CIS group required when CITYP=CIS required when CITYP=SFCIS The CIS method (singly excited CI) is the simplest way to treat excited states. By Brillouin's Theorem, a single determinant reference such as RHF will have zero matrix elements with singly substituted determinants. The ground state reference therefore has no mixing with the excited states treated with singles only. Reading the references given in Section 4 of this manual will show the CIS method can be thought of as a non-correlated method, rigorously so for the ground state, and effectively so for the various excited states. Some issues making CIS rather less than a black box method are: a) any states characterized by important doubles are simply missing from the calculation. b) excited states commonly possess Rydberg (diffuse) character, so the AO basis used must allow this. c) excited states often have different point group symmetry than the ground state, so the starting geometries for these states must reflect their actual symmetry. d) excited state surfaces frequently cross, and thus root flipping may very well occur. The normal CIS implementation allows the use of only RHF references, but can pick up both singlet and triplet excited states. Nuclear gradients are available, as are properties. The CIS run automatically includes computation of the dipole moments of all states, and all pairwise transition dipoles and oscillator strengths. The spin-flip type of CIS is very similar to spin-flip TD- DFT (the $TDDFT input contains more information about how spin-flip runs select the target state's Ms by $CONTRL's MULT value). The reference state must be UHF or ROHF, with MULT in $CONTRL at least 3. The target states of the CIS have one lower Ms, after one alpha spin in the reference is flipped to beta. Nuclear gradients are possible. Solvent effects are not available for either CIS or SFCIS. It is worthwhile to look at the $TDDFT input, which is a very similar calculation. The TD-DFT program offers the possibility of recovering some of the correlation energy, permits some solvent models, and can be used for MEX/CONICL surface intersection searches. The first six keywords are chemically important, while the remainder are mostly technical. NACORE = n Omits the first n occupied orbitals from the calculation (frozen core approximation). For CITYP=CIS, the default for n is the number of chemical core orbitals. For CITYP=SFCIS, the default, which is also the only possibility, is 0. NSTATE = Number of states to be found (excluding the reference state). No default is provided. IROOT = State for which properties and/or gradient will be calculated. Only one state can be chosen. The reference state is referred to as 0, and in the case of CITYP=SFCIS, might have a higher energy than some of the NSTATE target states. CISPRP = Flag to request the determination of CIS level properties, using the relaxed density. Relevant to RUNTYP=ENERGY jobs, although the default is .FALSE. because additional CPHF calculation will be required. Properties are an automatic by- product of runs involving the CIS or SFCIS nuclear gradient. HAMTYP = Type of CI Hamiltonian to use, if CITYP=CIS. = SAPS spin-adapted antisymmetrized product of the desired MULT will be used (default) = DETS determinant based, so both singlets and triplets will be obtained. MULT = Multiplicity (1 or 3) of the singly excited SAPS (the reference can only be singlet RHF). Only relevant for SAPS-based CITYP=CIS run, as SFCIS controls the Ms for target states by the value of MULT in $CONTRL. - - - - - - - - - - - - DIAGZN = Hamiltonian diagonalization method. = DAVID use Davidson diagonalization. (default) = FULL construct the full matrix in memory and diagonalize, thus determining all states (not recommended except for small cases). DGAPRX = Flag to control whether approximate diagonal elements of the CIS Hamiltonian (based only on the orbital energies) are used in the Davidson algorithm. Note, this only affects the rate of convergence, not the resulting final energies. If set .FALSE., the exact diagonal elements are determined and used. Default=.TRUE. NGSVEC = Dimension of the Hamiltonian submatrix that is diagonalized to form the initial CI vectors. The default is the greater of NSTATE*2 and 10. MXVEC = Maximum number of expansion basis vectors in the iterative subspace during Davidson iterations, before the expansion basis is truncated. The default is the larger of 8*NSTATE and NGSVEC. NDAVIT = Maximum number of Davidson iterations. Default=50. DAVCVG = Convergence criterion for Davidson eigenvectors. Eigenvector accuracy is proportional to DAVCVG, while the energy accuracy is proportional to its square. The default is 1.0E-05. CHFSLV = Chooses type of CPHF solver to use. = CONJG selects an ordinary preconditioned conjugate gradient solver. (default) = DIIS selects a diis-like iterative solver. RDCISV = Flag to read CIS vectors from a $CISVEC input group in the input file. Default is .FALSE. MNMEDG = Flag to force the use of the minimal amount of memory in construction of the CIS Hamiltonian diagonal elements. This is only relevant when DGAPRX=.FALSE., and is meant for debug purposes. The default is .FALSE. MNMEOP = Flag to force the use of the minimal amount of memory during the Davidson iterations. This is for debug purposes. The default is .FALSE. ========================================================== $CISVEC group required if RDCISV in $CIS is chosen This is formatted data generated by a previous CIS run, to be read back in as starting vectors. Sometimes molecular orbital phase changes make these CI vectors problematic. ==========================================================

generated on 7/7/2017