We show that the floating occupation molecular orbital complete active space configuration interaction (FOMO-CASCI) method is a promising alternative to the widely used complete active space self-consistent field (CASSCF) method in direct nonadiabatic dynamics simulations. We have simulated photodynamics of three archetypal molecules in photodynamics: ethylene, methaniminium cation, and malonaldehyde. We compared the time evolution of electronic populations and reaction mechanisms as revealed by the FOMO-CASCI and CASSCF approaches. Generally, the two approaches provide similar results. Some dynamical differences are observed, but these can be traced back to energetically minor differences in the potential energy surfaces. We suggest that the FOMO-CASCI method represents, due to its efficiency and stability, a promising approach for direct ab initio dynamics in the excited state.

We have calculated the absorption spectrum of dichlorine peroxide (ClOOCl) in the spectral range 250-400 nm. We have employed five different approaches to quantitatively model the absorption cross section (empirical broadening scheme, linearized harmonic reflection principle and full reflection principle with ground state density calculated using harmonic approximation, classical molecular dynamics, and path-integral molecular dynamics). We have also tested various single and multireference methods. We found that (i) the excitation characteristics of ClOOCl are sensitive on molecular geometries and therefore the ground state density has to be properly sampled and that (ii) single-reference methods tend to overestimate the absorption cross section and lead to a blueshift in the absorption maximum. The absorption spectrum calculated at the CASPT2 level with complete active space-configuration interaction (CAS-SCF) transition dipole moment with the ground state sampled via path-integral molecular dynamics is in a good agreement with the experiment. We, however, did not reach full agreement in the atmospherically relevant low-energy tail of the spectrum. The general message is that quantitative predictions are difficult even for a relatively small molecule as ClOOCl and the convergence, with respect to the electronic structure method, basis set used, and ground state sampling, needs to be carefully examined.

The photodissociation of imidazole in hydrogen bonded clusters has been studied at photodissociation wavelengths 243 and 193 nm. Imidazole clusters of different mean cluster sizes n approximately 3 and 6 have been produced in expansions with He and Ar carrier gases, and the mean cluster sizes were determined by mass spectrometric and crossed beam scattering experiments. Simultaneously, the (C(3)N(2)H(4))(n) clusters were studied by ab initio calculations for n up to 4 molecules, confirming the hydrogen bond N-H...N motif in the clusters. The measured H-fragment kinetic energy distribution spectra exhibit a bimodal character similar to the KEDs found for the bare molecule. (1) At 243 nm the fast H-atoms originate from the direct dissociation process on the repulsive pi sigma* state, and the slow component results from the dynamics populating the vibrationally hot ground state via an S(1)/S(0) conical intersection. In the clusters the contribution of the slow component increases with the cluster size. The slow component is also dominant at the shorter wavelength of 193 nm, where the dynamics starts with the excitation of pi pi* state. It is shown that the slow component in our experiment is a product of subsequent two-photon absorption. We have proposed different mechanisms how the observed enhanced internal conversion can be rationalized. The increased stability with respect to the H-fragment dissociation in clusters can be caused either by hydrogen transfer in the N-H...N bond or by closing the pi sigma* dissociation channel as in the case of pyrrole clusters.

We have investigated the photodynamics of phenol molecules in clusters. Possible reaction pathways following the photoexcitation of hydrogen-bonded phenol clusters have been identified theoretically using ab initio calculations. Experimentally we have studied the phenol molecules and clusters of various size distributions in a molecular beam apparatus. In particular, we have measured the H-fragment kinetic energy distributions after the excitation with 243 nm and 193 nm laser radiation. At 243 nm the KED spectra did not show any significant difference between the photodissociation of isolated molecules and phenol in larger clusters, while at 193 nm the contribution of the fast H-fragments is significantly suppressed in clusters with respect to the bare phenol molecule. We have interpreted the experimental results within the framework of the suggested reaction pathways.

The gas-phase photoelectron spectra of ethene, formaldehyde, formic acid and difluoromethane are simulated using the reflection principle and the unrestricted second-order algebraic diagrammatic construction [UADC(2)] scheme of the polarization propagator for the computation of the vertical-excited states of the cations at the equilibrium geometry of the parent neutral molecule. Comparison is made with experimental spectra and the established highly accurate ionization IP-ADC(3) theory to gain insight into the accuracy and applicability of recently developed excitation UADC schemes. Within UADC(2), we distinguish between the strict and extended schemes UADC(2)-s and UADC(2)-x. While the latter approach is found to slightly underestimate the experimental photoelectron spectra by 0.3 eV and can thus be regarded as a reliable scheme within the limits of the applied reflection principle and the underlying approximations, the UADC(2)-s scheme tends to overestimate the excitation energies by about 0.5 eV. Time-dependent density functional theory is also applied in combination with the standard B3LYP xc functional and turns out to be a useful computational tool for the simulation of the photoelectron spectra of the studied species.