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In this work we present computed high-harmonic generation (HHG) spectra of uracil and thyminemolecules, by means of the real-time time-dependent formulation of Gaussian-based configurationinteraction with single excitations (RT-TD-CIS). According to the experimental work [Hutchinsonetal., Phys. Chem. Chem. Phys. Comparison of high-order harmonic generation in uracil and thymineablation plumes, 2013,15, 12308] a pulse wavelength of 780 nm has been used, together with anintensity of 1014W/cm2and a pulse duration of 23 optical cycles. In order to examine the effect ofpulse polarisation, rotationally-averaged (to mimic the gas-phase sample) and single-polarisation havebeen computed for both molecules. Our results show that the HHG signal for both molecules possiblyoriginates from different ionisation channels, involving HOMO, HOMO-1, HOMO-2 and HOMO-3orbitals, which lie within 4 eV. We characterize the HHG spectrum of thymine, supporting the ideathat the absence of thymine signal in the original work does not depend on the single-moleculebehaviour. Present results for uracil are consistent with the experimental data. Moreover, we haveobserved that states below and above the chosen ionisation threshold provide different contributionsto the HHG spectrum in averaged and single-polarisation calculations

The penta-2,4-dieniminium cation (PSB3) displays similar ground state and first excited state potential energy features as those of the retinal protonated Schiff base (RPSB) chromophore in rhodopsin. Recently, PSB3 has been used to benchmark several electronic structure methods, including highly correlated multireference wave function approaches, highlighting the necessity to accurately describe the electronic correlation in order to obtain reliable properties even along the ground state (thermal) isomerization paths. In this work, we apply two quantum Monte Carlo approaches, the variational Monte Carlo and the lattice regularized diffusion Monte Carlo, to study the energetics and electronic properties of PSB3 along representative minimum energy paths and scans related to its thermal cis-trans isomerization. Quantum Monte Carlo is used in combination with the Jastrow antisymmetrized geminal power ansatz, which guarantees an accurate and balanced description of the static electronic correlation thanks to the multiconfigurational nature of the antisymmetrized geminal power term, and of the dynamical correlation, due to the presence of the Jastrow factor explicitly depending on electron-electron distances. Along the two ground state isomerization minimum energy paths of PSB3, CASSCF calculations yield wave functions having either charge transfer or diradical character in proximity of the two transition state configurations. Here, we observe that at the quantum Monte Carlo level of theory, only the transition state with charge transfer character can be located. The conical intersection, which becomes highly sloped, is observed only if the path connecting the two original CASSCF transition states is extended beyond the diradical one, namely by increasing the bond-length-alternation (BLA). These findings are in good agreement with the results obtained by MRCISD+Q calculations, and they demonstrate the importance of having an accurate description of the static and dynamical correlation when studying isomerization and transition states of conjugated systems. © 2015 American Chemical Society.

In this letter, we report the singlet ground state structure of the full carotenoid peridinin by means of variational Monte Carlo (VMC) calculations. The VMC relaxed geometry has an average bond length alternation of 0.1165(10) Å, larger than the values obtained by DFT (PBE, B3LYP, and CAM-B3LYP) and shorter than that calculated at the Hartree-Fock (HF) level. TDDFT and EOM-CCSD calculations on a reduced peridinin model confirm the HOMO-LUMO major contribution of the Bu(+)-like (S2) bright excited state. Many Body Green's Function Theory (MBGFT) calculations of the vertical excitation energy of the Bu(+)-like state for the VMC structure (VMC/MBGFT) provide an excitation energy of 2.62 eV, in agreement with experimental results in n-hexane (2.72 eV). The dependence of the excitation energy on the bond length alternation in the MBGFT and TDDFT calculations with different functionals is discussed.

Small (4)He clusters doped with a single atomic impurity, (1)H, (2)H, and (3)H, have been studied via a quantum Monte Carlo approach with the intent of establishing their binding behavior in nanoscopic clusters. Our calculations find that the only trimer (x)H (He)(2), which exhibits a bound state, is that with the tritium dopant (x = 3), in agreement with previous calculations using hyperspherical coordinates in the adiabatic approximation. The lightest dopant (1)H is seen not to stabilize the small helium clusters, while (2)H and (3)H are weakly bound to this solvent: our computed exchange energies and probability distribution functions reveal the "heliophobic" nature of hydrogen, leading thus to a further confirmation, in the realm of nanoscopic-size systems like the helium droplets, of the well-known nonmixing and nonsolvating features of hydrogen in macroscopic liquid bulk helium.

The structure and energetics of (3,4)HeCs(2)((3)Sigma(u)) molecules are analyzed from first principles. Fixing the cesium dimer at its equilibrium distance, the electronic structure was determined through ab initio methods at the CCSD(T) level of theory using a large basis set to compute the interaction energies. At the T-shaped geometry, there is a shallow well with a depth of approximately 2 cm(-1) placed at R approximately 6.75 A, R being the distance from the center of mass of Cs(2) to He. That depth gradually decreases to approximately 0.75 cm(-1), while R increases to about 11.5 A at linear arrangements. A simple model of adding atom-atom Lennard-Jones potentials with well-depth and equilibrium distance parameters depending on the angular orientation was found to accurately reproduce the ab initio points. Using this analytical form, variational calculations at zero total angular momentum are performed, predicting a single bound level at approximately -0.106 (approximately -0.042) cm(-1) for the boson (fermion) species. Further calculations using Quantum Monte Carlo methods are carried out and found to be in good agreement with the variational ones. On the basis of the present results, such analytical expression could in turn be used to describe the structure and binding of larger complexes and therefore opens the possibility to further studies involving such aggregates.

Ab initio computed interaction forces are employed in order to describe the microsolvation of the A$_2^+(^2\Sigma)$ (A=Li,Na,K) molecular ion in $^4$He clusters of small variable size. The minimum energy structures are obtained by performing energy minimization based on a genetic algorithm approach. The symmetry features of the collocation of solvent adatoms around the dimeric cation are analyzed in detail, showing that the selective growth of small clusters around the two sides of the ion during the solvation process is a feature common to all three dopants.