Enrico Tapavicza

Mixed time-dependent density-functional theory/classical trajectory surface hopping study of oxirane photochemistry

We present a mixed time-dependent density-functional theory (TDDFT)/classical trajectory surface hopping (SH) study of the photochemical ring opening in oxirane. Previous preparatory work limited to the symmetric CC ring-opening pathways of oxirane concluded that the Tamm-Dancoff approximation (TDA) is important for improving the performance of TDDFT away from the equilibrium geometry. This observation is supported by the present TDDFT TDA/SH calculations which successfully confirm the main experimentally derived Gomer-Noyes mechanism for the photochemical CO ring opening of oxirane and, in addition, provide important state-specific information not easily accessible from experiments. In particular, we find that, while one of the lowest two excited states is photochemically relatively inert, excitation into the other excited state leads predominantly to rapid ring opening, cyclic-C(2)H(4)O-->(*)CH(2)CH(2)O(*). This is followed by hopping to the electronic ground state where hot (4000 K) dynamics leads to further reactions, namely, (*)CH(2)CH(2)O()-->CH(3)CHO-->(*)CH(3)+(*)CHO and CH(4)+CO. We note that, in the dynamics, we are not limited to following minimum energy pathways and several surface hops may actually be needed before products are finally reached. The performance of different functionals is then assessed by comparison of TDDFT and diffusion Monte Carlo potential energy curves along a typical TDDFT TDA/SH reaction path. Finally, although true (S(0),S(1)) conical intersections are expected to be absent in adiabatic TDDFT, we show that the TDDFT TDA is able to approximate a conical intersection in this system.

Mixed time-dependent density-functional theory/classical photodynamics study of oxirane photochemistry

We present a mixed time-dependent density-functional theory (TDDFT)/classical trajectory surface hopping (SH) study of the photochemical ring opening in oxirane. Previous preparatory work limited to the symmetric CC ring-opening pathways of oxirane concluded that the Tamm-Dancoff approximation (TDA) is important for improving the performance of TDDFT away from the equilibrium geometry. This observation is supported by the present TDDFT TDA/SH calculations which successfully confirm the main experimentally derived Gomer-Noyes mechanism for the photochemical CO ring opening of oxirane and, in addition, provide important state-specific information not easily accessible from experiments. In particular, we find that, while one of the lowest two excited states is photochemically relatively inert, excitation into the other excited state leads predominantly to rapid ring opening, cyclic-C(2)H(4)O-->(*)CH(2)CH(2)O(*). This is followed by hopping to the electronic ground state where hot (4000 K) dynamics leads to further reactions, namely, (*)CH(2)CH(2)O()-->CH(3)CHO-->(*)CH(3)+(*)CHO and CH(4)+CO. We note that, in the dynamics, we are not limited to following minimum energy pathways and several surface hops may actually be needed before products are finally reached. The performance of different functionals is then assessed by comparison of TDDFT and diffusion Monte Carlo potential energy curves along a typical TDDFT TDA/SH reaction path. Finally, although true (S(0),S(1)) conical intersections are expected to be absent in adiabatic TDDFT, we show that the TDDFT TDA is able to approximate a conical intersection in this system.

Weakly bonded complexes of aliphatic and aromatic carbon compounds described with dispersion corrected density functional theory

Interaction energies and structural properties of van der Waals complexes of aliphatic hydrocarbons molecules and crystals of aromatic hydrocarbon compounds are studied using density functional theory augmented with dispersion corrected atom centered potentials (DCACPs). We compare the performance of two sets of DCACPs, (a) DCACP-MP2, a correction for carbon only, generated using MP2 reference data and a penalty functional that includes only equilibrium properties and (b) DCACP-CCSD(T), a set that has been calibrated against CCSD(T) reference data using a more elaborate penalty functional that explicitly takes into account some long-range properties and uses DCACP corrections for hydrogen and carbon atoms. The agreement between our results and high level ab initio or experimental data illustrates the transferability of the DCACP scheme for the gas and condensed phase as well as for different hybridization states of carbon. The typical error of binding energies for gas-phase dimers amounts to 0.3 kcal/mol. This work demonstrates that only one DCACP per element is sufficient to correct for weak interactions in a large variety of systems, irrespective of the hybridization state.

Quantum mechanical/molecular mechanical (QM/MM) car-parrinello simulations in excited states

The combination of time-dependent d. functional theory (TDDFT) for the description of excited states with a hybrid quantum mechanics/mol. mechanics (QM/MM) approach enables the study of photochem. processes in complex environments. Here, we present a short overview of recent applications of TDDFT/MM approaches to a variety of systems including studies of the optical properties of prototypical org. and inorg. mols. in gas phase and soln., photoinduced electron transfer reactions in donor-bridge-acceptor complexes, and in situ investigations of the mol. mechanisms of photoactive proteins. The application of TDDFT/MM techniques to a wide range of systems enables an assessment of the current performance and limitations of these methods for the characterization of photochem. processes in complex systems. [on SciFinder (R)]

Ab Initio Excited State Properties and Dynamics of a Prototype σ-Bridged-Donor−Acceptor Molecule

The photophysical and dynamical properties of the donor-(sigma-bridge)-acceptor molecule N-phenylpiperindone-malondinitrile are investigated by second-order approximate coupled cluster (CC2) and time-dependent density functional theory (TDDFT). The study is based on optimized equilibrium geometries for ground and excited states as well as on ab initio molecular dynamics simulations. While CC2 and DFT both predict ground state geometries that are consistent with the crystal structure, equilibrium geometries for the fluorescent charge transfer (CT) state are qualitatively different between CC2 and TDDFT. CC2 reproduces the experimental results for vertical excitations (within 0.3 eV) and provides an orbital assignment of the experimental absorption bands that is supported by experiments. Using CC2, a good agreement is also found for fluorescence energies (within 0.1-0.6 eV). At contrast, CT absorption and fluorescence energies are strongly underestimated by TDDFT using the semi-local functional PBE but improved agreement is found for the hybrid functional PBE0. However, for both functionals, TDDFT fails to predict an equilibrium geometry of the intradonor excited state because of mixing between this state and an artificially low-lying CT state during the optimization. This is an example where the well documented CT failure of TDDFT affects properties of other locally excited states. The minimum of the intradonor locally excited state was therefore only located by the CC2 method. The internal conversion (IC) process from a locally excited donor state to the CT state is simulated by excited state ab initio molecular dynamics based on CC2 and where nonadiabatic transitions are described using the Landau-Zener approximation. We find the IC process to occur a few tens of femtoseconds after excitation. The simulation provides a detailed description of the atomic rearrangements in electron donor and acceptor that drive the interconversion process.

Cyclohexadiene Revisited: A Time-Resolved Photoelectron Spectroscopy and ab Initio Study

We have reinvestigated the excited state dynamics of cyclohexa-1,3-diene (CHD) with time-resolved photoelectron spectroscopy and fewest switches surface hopping molecular dynamics based on linear response time-dependent density functional theory after excitation to the lowest lying ππ* (1B) state. The combination of both theory and experiment revealed several new results: First, the dynamics progress on one single excited state surface. After an incubation time of 35 ± 10 fs on the excited state, the dynamics proceed to the ground state in an additional 60 ± 10 fs, either via a conrotatory ring-opening to hexatriene or back to the CHD ground state. Moreover, ring-opening predominantly occurs when the wavepacket crosses the region of strong nonadiabatic coupling with a positive velocity in the bond alternation coordinate. After 100 fs, trajectories remaining in the excited state must return to the CHD ground state. This extra time delay induces a revival of the photoelectron signal and is an experimental confirmation of the previously formulated model of two parallel reaction channels with distinct time constants. Finally, our simulations suggest that after the initially formed cis-Z-cis HT rotamer the trans-Z-trans isomer is formed, before the thermodynamical equilibrium of three possible rotamers is reached after 1 ps.