Maurizio Persico

Direct semiclassical simulation of photochemical processes with semiempirical wave functions

We describe a new method for the simulation of excited state dynamics, based on classical trajectories and surface hopping, with direct semiempirical calculation of the electronic wave functions and potential energy surfaces ~DTSH method!. Semiempirical self-consistent-field molecular orbitals ~SCF MO’s! are computed with geometry-dependent occupation numbers, in order to ensure correct homolytic dissociation, fragment orbital degeneracy, and partial optimization of the lowest virtuals. Electronic wave functions are of the MO active space configuration interaction ~CI! type, for which analytic energy gradients have been implemented. The time-dependent electronic wave function is propagated by means of a local diabatization algorithm which is inherently stable also in the case of surface crossings. The method is tested for the problem of excited ethylene nonadiabatic dynamics, and the results are compared with recent quantum mechanical calculations.

Newton-X: a surface-hopping program for nonadiabatic molecular dynamics

The Newton‐X program is a general‐purpose program package for excited‐state molecular dynamics, including nonadiabatic methods. Its modular design allows Newton‐X to be easily linked to any quantum‐chemistry package that can provide excited‐state energy gradients. At the current version, Newton‐X can perform nonadiabatic dynamics using Columbus, Turbomole, Gaussian, and Gamess program packages with multireference configuration interaction, multiconfigurational self‐consistent field, time‐dependent density functional theory, and other methods. Nonadiabatic dynamics simulations with a hybrid combination of methods, such as Quantum‐Mechanics/Molecular‐Mechanics, are also possible. Moreover, Newton‐X can be used for the simulation of absorption and emission spectra. The code is distributed free of charge for noncommercial and nonprofit uses at www.newtonx.org. WIREs Comput Mol Sci 2014, 4:26–33. doi: 10.1002/wcms.1158

Including quantum decoherence in surface hopping.

In this paper we set up a method called overlap decoherence correction (ODC) to take into account the quantum decoherence effect in a surface hopping framework. While keeping the standard surface hopping approach based on independent trajectories, our method allows to account for quantum decoherence by evaluating the overlap between frozen Gaussian wavepackets, the time evolution of which is obtained in an approximate way. The ODC scheme mainly depends on the parameter σ, which is the Gaussian width of the wavepackets. The performance of the ODC method is tested versus full quantum calculations on three model systems, and by comparison with full multiple spawning (FMS) results for the S(1)→S(0) decay in the azobenzene molecule.

Photodynamics and Time-Resolved Fluorescence of Azobenzene in Solution: A Mixed Quantum-Classical Simulation

We have simulated the photodynamics of azobenzene by means of the Surface Hopping method. We have considered both the trans → cis and the cis → trans processes, caused by excitation in the n → π* band (S(1) state). To bring out the solvent effects on the excited state dynamics, we have run simulations in four different environments: in vacuo, in n-hexane, in methanol, and in ethylene glycol. Our simulations reproduce very well the measured quantum yields and the time dependence of the intensity and anisotropy of the transient fluorescence. Both the photoisomerization and the S(1) → S(0) internal conversion require the torsion of the N═N double bond, but the N-C bond rotations and the NNC bending vibrations also play a role. In the trans → cis photoconversion the N═N torsional motion and the excited state decay are delayed by increasing the solvent viscosity, while the cis → trans processes are less affected. The analysis of the simulation results allows the experimental observations to be explained in detail, and in particular the counterintuitive increase of the trans → cis quantum yield with viscosity, as well as the relationship between the excited state dynamics and the solvent effects on the fluorescence lifetimes and depolarization.

An ab initio study of the photochemistry of azobenzene.

The photochemistry under n→π* and π→π* excitation and the thermal isomerization of azobenzene are studied by a theoretical approach. Two mechanisms are considered: torsion around the N2N double bond and the inversion of one N atom. We optimize the most important geometries with CASSCF and we calculate the potential energy curves for the lowest states (4 singlets and 2 triplets) with multireference perturbation theory. Our calculations support the current view that inversion is the preferred pathway for ground state isomerization and probably also by n→π* excitation, while torsion occurs by π→π* excitation. We find that two states, 21A and 21B in C2 symmetry, are important in the π→π* photochemistry.

Quantum mechanical and semiclassical dynamics at a conical intersection

We present simulations of wave‐packet dynamics for a model of a conical intersection in two dimensions. The potential energy surfaces and couplings are functions of a total symmetrical coordinate and of a symmetry breaking one. The wave packet crosses the coupling region once, moving essentially in the direction of the symmetrical coordinate. The dynamics are determined by two methods, one quantum mechanical and the other semiclassical, based on trajectories and surface hopping. The semiclassical approximation is quite adequate for low coupling strengths in the diabatic representation, less so for larger couplings. Approximate analytic solutions for the two‐dimensional problem and for one‐dimensional analogs are provided, in order to generalize the numerical results and to analyze the reasons of the discrepancies between semiclassical and quantum mechanical results.

Surface hopping dynamics using a locally diabatic formalism: Charge transfer in the ethylene dimer cation and excited state dynamics in the 2-pyridone dimer

In this work, the advantages of a locally diabatic propagation of the electronic wave function in surface hopping dynamics proceeding on adiabatic surfaces are presented providing very stable results even in challenging cases of highly peaked nonadiabatic interactions. The method was applied to the simulation of transport phenomena in the stacked ethylene dimer radical cation and the hydrogen bonded 2-pyridone dimer. Systematic tests showed the reliability of the method, in situations where standard methods relying on an adiabatic propagation of the wave function and explicit calculation of the nonadiabatic coupling terms exhibited significant numerical instabilities. Investigations of the ethylene dimer radical cation with an intermolecular distance of 7.0 Å provided a quantitative description of diabatic charge trapping. For the 2-pyidone dimer, a complex dynamics was obtained: a very fast (<10 fs) initial S(2)∕S(1) internal conversion; subsequent excitation energy transfers with a characteristic time of 207 fs; and the occurrence of proton coupled electron transfer (PCET) in 26% of the trajectories. The computed characteristic excitation energy transfer time of 207 fs is in satisfactory agreement with the experimental value of 318 fs derived from the vibronic exciton splittings in a monodeuterated 2-pyridone dimer complex. The importance of nonadiabatic coupling for the PCET related to the electron transfer was demonstrated by the dynamics simulations.

Surface hopping trajectory simulations with spin-orbit and dynamical couplings

In this paper we consider the inclusion of the spin-orbit interaction in surface hopping molecular dynamics simulations to take into account spin forbidden transitions. Two alternative approaches are examined. The spin-diabatic one makes use of eigenstates of the spin-free electronic Hamiltonian and of Ŝ(2) and is commonly applied when the spin-orbit coupling is weak. We point out some inconsistencies of this approach, especially important when more than two spin multiplets are coupled. The spin-adiabatic approach is based on the eigenstates of the total electronic Hamiltonian including the spin-orbit coupling. Advantages and drawbacks of both strategies are discussed and illustrated with the help of two model systems.

Competing ultrafast intersystem crossing and internal conversion: a time resolved picture for the deactivation of 6-thioguanine

In this paper we simulate the deactivation dynamics of photoexcited 6-thioguanine, a cytotoxic analogue of the canonical DNA/RNA base guanine, using a direct surface hopping dynamics approach. Our aim is to investigate the mechanism for triplet population, which was found to take place on a similar time scale as internal conversion. The surface hopping calculations were based on potential energy surfaces and couplings obtained on the fly using a semiempirical Hamiltonian, reparameterized on accurate ab initio data. We show that for the full description of the deactivation dynamics of 6-thioguanine, it is important to take into account both the dynamic and the spin–orbit couplings. The main deactivation pathway involves the sequence of ultrafast radiationless transitions S2 → S1 → T2 → T1. The very efficient population and long lifetime of the final T1 state, from where singlet oxygen is generated, would explain the high phototoxicity of the nucleotides of 6-thioguanine in DNA. To our knowledge, this is the first nonadiabatic dynamics simulation for a system showing strong spin–orbit couplings (due to the presence of a third row atom, sulfur) and a complex pattern of intermultiplet crossings.

Efficient calculation of Franck–Condon factors and vibronic couplings in polyatomics

We present a technique for the calculation of Franck–Condon factors and other integrals between vibronic wave functions belonging to different electronic states. The technique is well suited for the determination of the nonadiabatic or spin‐orbit couplings related to radiationless decays in polyatomics. Rigorous or approximate partitions of the internal coordinate space are exploited to achieve better efficiency and/or to go beyond the harmonic approximation. The technique is tested by computing the Internal Conversion and InterSystem Crossing rates of (CH3)3CNO in its 1(n→π*) state.