A three-dimensional intermolecular potential energy surface of the Ar-CO complex has been determined by utilizing previously reported spectroscopic data, where 971 transition frequencies by microwave, millimeter-wave, submillimeter-wave, and IR spectroscopy were reproduced simultaneously within experimental accuracies. A free rotor model Hamiltonian considering all the freedom of motions for an atom-diatom system was used to calculate vibration-rotation energies. The three-dimensional potential energy surface by ab initio calculations at the CCSD(T)-F12b/aug-cc-pV5Z level of theory were parameterized by a model function consisting of 46 potential parameters and they were used as initial values in the least-squares analysis. In total 20 potential parameters were optimized to reproduce all the experimental data.

The C₃Ar and C₃Ne complexes have four large amplitude vibrations. These are the in- and out-of-plane C₃ bending motions, and the two van der Waals vibrations. Assignment of the spectra of the complexes is therefore challenging. The ab initio potential energies of their ground electronic states have been calculated at the CCSD(T)/pVQZ level. 46620 points have been computed to describe the four-dimensional potential of C₃Ar: ∠C−C−C = 112-179.5^°, r (C-C bond length) = 1.298 Å, R(C₃-Ar) = 3.4-6.0 Å, ϕ (azimuth angle between Ar and the principal axis of C₃)=0-180^°, and θ (colatitude angle) = 0-180^°. For C₃Ne, 69190 points have been computed: ∠C−C−C=106-179.5^°, r(C-C)= 1.298Å, R(C₃-Ne) = 3.0-7.0 Å, ϕ = 0-180^°, θ = 0-180^°. Basis set superposition errors have been corrected in the C₃-Ne potential energy calculations. Morse type functions and power series were used to fit the calculated points.

The Heidelberg multiconfiguration time-dependent Hartree package¹ was used to calculate the vibrational level structures of the ground electronic states of the C₃-Ar and C₃-Ne complexes. The previously reported 4-D ab initio potentials were converted to 6-D potentials by adding the potential energies of the C-C symmetric and antisymmetric stretching vibrations of C₃. They were subsequently transformed from internal coordinates to Jacobi coordinates. The kinetic-energy operators were taken from Yang and Kühn². Preliminary results show that large amplitude motions occur in five coordinates: C-C-C bond angle, out-of-plane tilt angle, van der Waals stretch, van der Waals bend and one of the C-C bonds.
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¹G.A. Worth, M.H. Beck, A. Jäckle, H.-D. Meyer, F. Otto, M. Brill, and O. Vendrell, The MCTDH package, version 8.4, Heidelberg University, Heidelberg, Germany, 2011. ²Y. Yang and O. Kühn, Mol. Phys. 106, 2445(2008)

The rich spectroscopy of the ethyl radical has attracted the attention of several experimental and theoretical investigations. Its molecular spectrum contains signatures of hyperconjugation, torsion-inversion coupling, and Fermi coupling. We present a full-dimensional theoretical treatment of this vibrational problem using a combination of Van Vleck perturbation theory and variational approaches to further explore these effects. A CCSD(T)/cc-pVTZ potential energy surface and a numerical computation of the kinetic energy operator are employed to construct the Hamiltonian. Our calculations use coordinates that exploit the system's G₁₂ PI symmetry to produce compact expressions for the potential energy. This symmetry also allows for a simplified evaluation of the Hamiltonian and greatly reduces the cost of diagonalization. For this talk we illustrate the Van Vleck treatment of the vibrational modes in the ethyl radical and how such a treatment accelerates basis set convergence of the resulting eigenvalues.

Two parallel theories including Franck–Condon, Herzberg–Teller and Duschinsky (i.e., mode mixing) effects, allowing different approximations for the description of excited state PES have been developed in order to simulate realistic, asymmetric, electronic spectra line-shapes taking into account the vibrational structure: the so-called sum-over-states or time-independent (TI) method and the alternative time-dependent (TD) approach, which exploits the properties of the Fourier transform. The integrated TI-TD procedure included within a general purpose QM code [1,2], allows to compute one photon absorption, fluorescence, phosphorescence, electronic circular dichroism, circularly polarized luminescence and resonance Raman spectra. Combining both approaches, which use a single set of starting data, permits to profit from their respective advantages and minimize their respective limits: the time-dependent route automatically includes all vibrational states and, possibly, temperature effects, while the time-independent route allows to identify and assign single vibronic transitions. Interpretation, analysis and assignment of experimental spectra based on integrated TI-TD vibronic computations will be illustrated for challenging cases of medium-sized open-shell systems in the gas and condensed phases with inclusion of leading anharmonic effects. 1. V. Barone, A. Baiardi, M. Biczysko, J. Bloino, C. Cappelli, F. Lipparini Phys. Chem. Chem. Phys, 14, 12404, (2012) 2. A. Baiardi, V. Barone, J. Bloino J. Chem. Theory Comput., 9, 4097–4115 (2013)

Accurate quantum-chemical calculations of metal quadrupole-coupling parameters are challenging due to the sensitivity of these parameters to both relativistic and electron-correlation effects. In the present study we have employed the spin-free exact two-component theory in its one-electron variant for a cost-effective treatment of scalar-relativistic effects in combination with coupled-cluster methods for a systematic incorporation of electron correlation. Spin-orbit effects have been included by means of perturbation theory at the Hartree-Fock self-consistent-field level. The accuracy and applicability of the model presented here is demonstrated with calculations of metal quadrupole-coupling parameters for a set of copper and gold compounds.

Recently interest in polar diatomic molecules with a magnetic dipole moment has been growing. An example for such molecules is the combination of an alkali metal atom and an alkaline earth metal atom. These systems are quite small, containing only three valence electrons. Nevertheless calculations of excited states are challenging. Ab initio calculations for two sample systems, LiCa¹ and RbSr, will be presented. The potential energy curves and transition dipole moments for the ground state and several excited states were determined, up to 25000 cm⁻¹for LiCa and up to 22000 cm⁻¹for RbSr. Multireference configuration interaction calculations (MRCI) based on complete active space self-consistent field wave functions (CASSCF) were used to determine the properties of the system as implemented in the MOLPRO software package.² Effective core potentials (ECPs) and core polarization potentials (CCPs) were applied to reduce the computational effort, while retaining accuracy. The similarities and differences of the two systems will be discussed. In both systems the accurate description of the asymptotic values of the PECs corresponding to atomic D-states proved to be difficult. The results will be compared to recent experiments, showing that a combination of theory and experiment gives a reliable description of the systems.
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¹G. Krois, J.V. Pototschnig, F. Lackner and W.E. Ernst, J. Phys. Chem. A, 117, 13719-13731 (2013) ²H.-J. Werner and P. J. Knowles and G. Knizia and F. R. Manby and M. Schütz et al., MOLPRO, version 2010.1, see http://www.molpro.net/

The discovery of a set of seven conical intersections (CI’s) between vibrationally adiabatic surfaces in methanol is reported. The intersecting surfaces represent the energies of the two asymmetric CH stretch vibrations, ν₂ and ν₉, regarded as adiabatic functions of the torsional angle, γ, and COH bend angle, ρ. One conical intersection, required by symmetry, is located at the C_3v geometry where the COH group is linear (ρ = 0^°); the other six are in eclipsed conformations with ρ = 62^° and 94^°. The three CI’s at ρ = 62^° are close to the equilibrium geometry (ρ = 71.4^°), within the zero-point amplitude of the COH bending vibration. CI’s between electronic surfaces have long been recognized as crucial conduits for ultrafast relaxation, and recently Hamm, and Stock^{1 2} have shown that vibrational CI’s may also provide a mechanism for ultrafast vibrational relaxation. The ab initio data reported here are well described by an extended Zwanziger and Grant model³ for E ⊗ e Jahn-Teller systems in which Renner-Teller coupling is also active. However, in the present case, the distortion ρ from C_3v symmetry is much larger than is typical in the Jahn-Teller coupling of electronic surfaces and accordingly higher-order terms in ρ are required. The present results are also consistent with the two-state model of Xu et al⁴. The cusp-like features, which they found along the internal-rotation path, are explained in the context of the present work in terms of proximity to the CI’s. The presence of multiple CI’s near the torsional minimum energy path impacts the role of geometric phase in this three-fold internal-rotor system. When the dimensionality of the low-frequency space is extended to include the CO bond length as well as γ and ρ, the individual CI’s become seams of CI’s. It is shown that the CI’s at ρ = 62^° and 94^° lie along the same seam of CI’s in this higher dimensional space.
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¹P. Hamm and G. Stock, Phys. Rev. Lett. 109, 173201, (2012). ²P. Hamm, and G. Stock, Mol. Phys. 111, 2046, (2013). ³J. W.  Zwanziger, and E. R. Grant, J. Chem. Phys. 87, 2954, (1987). ⁴L.-H. Xu, J. T.  Hougen, and R. M.  Lees, J. Mol. Spectrosc. 293-294, 38, (2013).

The ubiquitous nature of the low-lying πσ* state in the photo-excited aromatic molecules or biomolecules is widely recognized to play an important role in nonadiabatic photo-process such as photodissociation or intramolecular charge transfer (ICT). For instance, the O-H elimination channel in phenol is attributed to the state-cross of the repulsive πσ* state that exhibits a conical intersection with the lowest bright ππ* state and with the ground state, leading to ultrafast electronic deactivation. A similar decay pathway has been found in the ICT formation of 4-(dialkylamino)benzonitriles in a polar environment, where an initially photoexcited Frank-Condon state bifurcates in the presence of a dark intermediate πσ* state that crosses the fluorescent ππ* state, followed by a conical intersection with the twisted intramolecular charge transfer (TICT) state. We proposed such a two-fold decay mechanism that πσ*-state highly mediates intramolecular charge transfer in 4-(dialkylamino)benzonitriles, which is supported from both our high-level ab initio calculations and ultrafast laser spectroscopies in the previous study. 4-(Dimethylamino)benzethyne (DMABE) is isoelectronic with 4-(dimethylamino)benzonitrile (DMABN), and the electronic structures and electronic spectra of the two molecules bear very close resemblance. However, DMABN does show the ICT formation in a polar environment, whereas DMABE does not. To probe the photophysical differences among the low-lying excited-state configurations, we performed concerted time-resolved laser spectroscopies and high level ab initio multireference perturbation theory quantum-chemical (CASPT2//CASSCF) computations on the two molecules. In this paper we demonstrate the importance of the bound excited-state of a πσ* configuration that induce highly πσ*-state mediated intramolecular charge transfer in 4-(dialkylamino)benzonitriles.

We have carried out TDDFT computational studies on the low-lying excited states of di-tert-butylaminobenzonitrile and 2,4,6-tricyanoaniline compounds that exhibit unusual photophysical behaviors associated with the intramolecular charge transfer (ICT). For both 3- and 4-di-tert-butylamino)benzonitriles (m-DTBABN and p-DTBABN, respectively) show the ICT formation, and p-DTBABN appears to be the only meta-substituted aminobenzonitrile that exhibits the ICT formation. The TDDFT calculations indicate evidence that the ultrafast ICT formation in p-DTBABN and m-DTBABN is due to the sequential state switches: ππ^*(L_a)→ πσ^*→ ICT in the presence of conical intersections among the three closely-lying excited-states. On the other hand, 2,4,6-tricyanoaniline does not show clear evidence for the LE (locally excited) state → ICT state formation from steady-state fluorescence studies, despite the greater electron acceptor strength of tricycanobenzene as compared to monocyanobenzene, which is part of a 4-(dimethylamino)benzonitrile (p-DMABN) compound. However, it is predicted that 2,4,6-tricyano-N,N-dimethylaniline (TCDMA), but not 2,4,6-tricyanoaniline (TCA), possesses two ICT states, which show the ICT-characterized quinoidal structures and lie below the initially photo-excited S₁(ππ^*) state. The CC2 calculations further predict two conformers as labeled with quinoidal (ICT-Q) and anti-quinoidal (ICT-AQ) structures are rapidly interconnecting with each other. The lower energy ICT-Q structure tends to be populated from the unstable ICT-AQ structure, which is responsible for the observed time-resolved fluorescence as well as the excited-state absorption from the mixed S₁(ππ^*)/ICT state of TCDMA. In both cases for TCDMA and TCA, the πσ^* state locates significantly higher in energy than the S₁(ππ^*) state (and the ICT state for TCA), thus precluding the πσ^*→ ICT formation, which is believed to occur in a p-DMABN in polar environments.

New potential energy surfaces for methane were constructed, represented as analytic fits to about 100,000 individual high-level ab initio data. Explicitly-correlated multireference data (MRCI-F12(AE)/CVQZ-F12) were computed using Molpro [1] and fit using multiple strategies. Fits with small to negligible errors were obtained using adaptations of the permutation-invariant-polynomials (PIP) approach [2,3] based on neural-networks (PIP-NN) [4,5] and the interpolative moving least squares (IMLS) fitting method [6] (PIP-IMLS). The PESs were used in full-dimensional vibrational calculations with an exact kinetic energy operator by representing the Hamiltonian in a basis of products of contracted bend and stretch functions and using a symmetry adapted Lanczos method to obtain eigenvalues and eigenvectors. Very close agreement with experiment was produced from the purely ab initio PESs. References 1- H.-J. Werner, P. J. Knowles, G. Knizia, 2012.1 ed. 2012, MOLPRO, a package of ab initio programs. see http://www.molpro.net. 2- Z. Xie and J. M. Bowman, J. Chem. Theory Comput 6, 26, 2010. 3- B. J. Braams and J. M. Bowman, Int. Rev. Phys. Chem. 28, 577, 2009. 4- J. Li, B. Jiang and Hua Guo, J. Chem. Phys. 139, 204103 (2013). 5- S Manzhos, X Wang, R Dawes and T Carrington, JPC A 110, 5295 (2006). 6- R. Dawes, X-G Wang, A.W. Jasper and T. Carrington Jr., J. Chem. Phys. 133, 134304 (2010).