Watson's landmark reformulation of the Born-Oppenheimer separation problem has been the basis of most combined-isotopologue analyses of diatomic spectroscopic data since 1980.J.K.G. Watson, J. Mol.\ Spectrosc. 80, 411 (1980)ne noteworthy feature of this work was his proposal that description of the dynamical behaviour of a diatomic ion with (±) charge Q, formed from atoms with isotopic masses of M₁ and M₂, should use a "charge-modified" reduced mass:  μ_Wat ≡ M₁ M₂ /(M₁ + M₂ − Q m_e) , in which m_e is the electron mass, and this proposal seems to have been benignly accepted and adopted. The first quantitative test of this proposal was in the pioneering combined-isotopologue direct-potential-fit (CI-DPF) study of HeH⁺ by Coxon and Hajigeorgiou in 1999,J.A. Coxon and P. Hajigeorgiou, J. Mol. Spectrosc. 193, 306 (1999).here they compared the quality of fit for analyses that used different choices for the definition of the reduced masses of the various isotopologues, and found that the best choice seemed to be to use conventional two-body reduced masses for (M₁ − \frac1 2  m_e) and (M₂ − \frac12 m_e ). This question was re-examined recently in the context of a CI-DPF study of CH⁺, and a rather different conclusion was reached.Y.-S. Cho and R.J. Le Roy, J. Chem. Phys.\ 144, 024311 (2016).he present paper combines new CI-DPF studies of HeH⁺ and BeH⁺ with our recent CH⁺ work, and attempts to draw some general conclusions on this matter.

---

Footnotes:

J.K.G. Watson, J. Mol.\ Spectrosc. 80, 411 (1980)O J.A. Coxon and P. Hajigeorgiou, J. Mol. Spectrosc. 193, 306 (1999).w Y.-S. Cho and R.J. Le Roy, J. Chem. Phys.\ 144, 024311 (2016).T

Carbon monoxide is after molecular hydrogen the second most abundant molecule in the interstellar medium (ISM) and also an important molecule for processes occurring in the atmosphere, and hydrocarbon combustion. The rate coefficients of CO in collision with dominant species like H, H₂, He, etc are common keys to understand the CO emission spectrum or to model combustion chemistry processes. The inelastic scattering of H+CO has indeed been intensively studied in the past decades, using mainly the so-called WKS PES developed by Werner et al or recently a modified version by Song et al. Though the spectroscopic agreement of the WKS surface with experiment is known to be quite good, there is no experimental evidence that the dynamics of the system is correctly reproduced by the surface. We will present in this talk new results on a set of HCO surfaces of the ground and the excited Renner-Teller coupled electronic states with the principal objective of studying the influence of the Renner-Teller coupling on the inelastic scattering of H+CO. Our calculations done using the MCTDH algorithm cover the 0-2 eV energy range and allow one to interpret the effect of the Renner-Teller coupling on the rovibrational inelastic scattering. Additionally, vibrational bound and resonance state calculations on this new PES and comparisons with available experimental data will be presented.

We report 13 room temperature line lists for all major CO₂ isotopologues, covering 0-8000 cm⁻¹. These line lists are a response to the need for line intensities of high, preferably sub-percent, accuracy by remote sensing experiments. Our scheme encompasses nuclear motion calculations supported by critical reliability analysis of the generated line intensities. Rotation-vibration wavefunctions and energy levels are computed using DVR3D and a high quality semi-empirical potential energy surface (PES) [1], followed by computation of intensities using a fully ab initio dipole moment surface (DMS). Cross comparison of line lists calculated using pairs of high-quality PES's and DMS's is used to assess imperfections in the PES, which lead to unreliable transition intensities between levels involved in resonance interactions. Four line lists are computed for each isotopologue to quantify sensitivity to minor distortions of the PES/DMS. This provides an estimate of the contribution to the overall line intensity error introduced by the underlying PES. Reliable lines are benchmarked against recent state-of-the-art measurements [2] and HITRAN-2012 supporting the claim that the majority of line intensities for strong bands are predicted with sub-percent accuracy [3]. Accurate line positions are generated using an effective Hamiltonian [4]. We recommend use of these line lists for future remote sensing studies and inclusions in databases. px

[]

X. Huang, D. W. Schwenke, S. A. Tashkun, T. J. Lee, J. Chem. Phys. 136, 124311, 2012.

[]

O.  L. Polyansky, K. Bielska, M. Ghysels, L. Lodi, N.  F. Zobov, J.  T. Hodges, J. Tennyson, PRL, 114, 243001, 2015.

[]

E. Zak, J. Tennyson, O. L. Polyansky, L. Lodi, S. A. Tashkun, V. I. Perevalov, JQSRT, in press and to be submitted.

[]

S. A. Tashkun, V. I. Perevalov, R. R. Gamache, J. Lamouroux, JQSRT, 152, 45-73, 2015.

r0pt Figure Vibrational spectra of hydrogen bonded bimolecular complexes (XH…Y, where X is the hydrogen bond donor atom, and Y is the acceptor atom) have long been a theoretical challenge. Specifically, the XH-stretching motion is difficult to describe due to the effect of the large amplitude intermolecular modes inherent to complexes. We have developed a vibrational model, the Local Mode Perturbation Theory (LMPT) model, to accurately determine the transition wavenumber and oscillator strength of the XH-stretching transition in hydrogen bonded bimolecular complexes. The model is based on a local mode (LM) model of the XH-stretching transition and the effect of the intermolecular modes is included via Rayleigh-Schrödinger perturbation theory. Our model has significantly improved results obtained using the LM model (see Figure). Additionally, our LMPT model does not require a full-dimensional anharmonic calculation, which enables application to large systems and the usage of higher level ab initio theory for the required potential energy surfaces. This work was inspired by our recent efforts to accurately determine equilibrium constants of complex formation, which rely on an accurate determination of the oscillator strength of the XH-stretching transition.

Fucose is a deoxyhexose that is present in a wide variety of organisms. The stereochemical information, namely, glycosidic linkages α or β, gives significant features of the carbohydrate glycosidation position of the glycosylic acceptor [1].
Due to its applicability to all organic molecules and the reliability of ab initio quantum calculation, Vibrational Circular Dichroism VCD has some advantages over conventional electronic spectroscopy. However, for a molecule with many chiral centers such as carbohydrate, determination of the absolute configuration tends to be difficult because the information from each stereochemical center are mixed and averaged over the spectrum.
T. Taniguchi et al [2] reported that in the CH stretching region of carbohydrate, methyl glycosides exhibit a characteristic VCD peak at 2840 cm⁻¹, the sign solely reflects the anomeric carbon absolute configuration.
This work is an investigation of theoretical VCD spectra of α-L-fucopyranoside and β-L-fucopyranoside with an implicit (PCM) and explicit consideration of water molecules using density functional theory DFT and the Potential Energy Distribution's analysis (PED) .
Keywords : VCD, DFT, PED, Fucopyranoside .
[1]. a) C.-H. Wong, Carbohydrate –Based Drug Discovery, Wiley-VCH Weinheinium 2003; b) T. Taniguchi, K. Monde, Wiley VCH Verlag GmbH§ Co. KGaA, Weinhein chem asian J. 2007, 2,1258-1266 [2]. T. Taniguchi et al. Tetrahedron Letters 45 (2004) 8451–8453

Being the simplest uncharged homonuclear dimer after H₂ that has a stable ground state, Li₂ is one of the most important benchmark systems for theory and experiment. In 1930, Delbrück used Li₂ to test his theory of homopolar binding, and it was used again and again as a prototype to test what have now become some of the most ubiquitous concepts in molecular physics (LCAO, SCF, MO, just to name a few). Experimentally, Roscoe and Schuster studied alkali dimers back in 1874. At the dawn of quantum mechanics, the emerging types of spectroscopic analyses we now use today, were tested on Li₂ in the labs of Wurm (1928), Harvey (1929), Lewis (1931), and many others, independently. Li₂ was at the centre of the development of PFOODR in the 80s, and PAS in the 90s; and Lithium Bose-Einstein condensates were announced only 1 month after the Nobel Prize winning BEC announcement in 1995. Even now in the 2010s, numerous experimental and theoretical studies on Li have tested QED up to the 7th power of the fine structure constant. Li₂ has also been of interest to sub-atomic physicists, as it was spectroscopic measurements on ⁷Li₂ that determined the spin of ⁷Li to be 3/2 in 1931; and Li₂ has been proposed in 2014 as a candidate for the first "halo nucleonic molecule". The lowest triplet state a(1³Σ_u⁺) is an excellent benchmark system for all newly emerging ab initio techniques because it has only 6e⁻, its potential is only 334 cm⁻¹ deep, it avoids harsh complications from spin-orbit coupling, and it is the deepest potential for which all predicted vibrational energy levels have been observed with 0.0001 cm⁻¹ precision. However the current best ab initio potentials do not even yield all vibrational energy spacings correct to within 1 cm⁻¹. This could be because the calculation was only done on a cc-pV5Z basis set, or because the QCISD(T,full) method that the authors used, only considered triple excitations while a full CI calculation should include up to hexuple excitations. CCSDTQPH calculations have never yet been reported for anything larger than a DZ basis set, and deterministic FCI calculations for 6e⁻ have not exceeded the level of TZ basis sets. With FCIQMC and DMRG we are able to calculate the potential with all levels of excitation included, and the hardware requirements for an aug-cc-pCV6Z basis set are modest. Energies for aug-cc-pCVQZ have already converged to the full CI limit within 0.3 cm⁻¹, and 6Z potentials are underway.

The ground electronic state of the chromium dimer dissociates into Cr (⁷S) + Cr (⁷S) and therefore the fragments are highly open shell systems with a total of 12 singly occupied orbitals among its constituent atoms. It is considered one of the most difficult homonuclear diatomics for ab initio methods because of its highly multi-reference character. Therefore, every new multi-reference method must be tested against this benchmark system. However, the best empirical potential to compare with, has its own weaknesses. The photoelectron measurements of v=0−9 were fitted to a Morse potential (an old function which has only one parameter controlling the shape from r_e to D_e), and also inverted using a semi-classical theory into a potential after combining these data with measurements from what were hypothesized to be v=24−43. This bridging of a  ∼  2000 cm⁻¹ gap in data back in 1993 was a valiant spectroscopic analysis. However since 1993, there have been enormous improvements in the field of potentiology. In 2011 a Morse/long-range (MLR) function successfully bridged a gap of more than 5000 cm⁻¹ in experimental data^a, and in 2013 an experiment with ±0.000 02 cm⁻¹ resolution confirmed that the 2011 MLR predicted the energy levels in the very center of this gap correctly within  ∼  1 cm⁻¹,^b. While ab initio methods have very recently been able to predict differences in energy levels correctly to within 1 cm⁻¹ for Li₂ ^c and to a lesser extent for BeH^d, ab initio methods have still not had this level of success for predicting binding energies. The MLR function not only has more flexibility than the original Morse function, but it also converges mathematically to the correct long-range limit expected by the state-of-the-art theory. Fitting the data to an MLR potential function in the Schrödinger equation allows for a fully quantum mechanical treatment over the entire range of data. By avoiding a semi-classical treatment, and using this more flexible, more theoretically correct form, we improve the current best empirical potential. This vastly improves the experimental benchmarks against which emerging ab initio methods are tested. However, the lack of data for Cr₂ is still a big problem, so further experimental work on Cr₂ is desperately needed.

Research on ultracold molecules has seen a growing interest recently in the context of high-resolution spectroscopy and quantum computation. The preparation of molecules in low vibrational levels of the ground state is experimentally challenging, and typically achieved by population transfer using excited electronic states. On the theoretical side, highly accurate potential energy surfaces are needed for a correct description of processes such as the coherent de-excitation from the highest and therefore weakly bound vibrational levels in the electronic ground state via couplings to electronically excited states. Particularly problematic is the correct description of potential features at large intermolecular distances. Franck-Condon overlap integrals for nuclear wavefunctions in barely bound vibrational states are extremely sensitive to inaccuracies of the potential at long range. In this study, we compare the predictions of common, wavefunction-based ab initio techniques for a known de-excitation mechanism in alkali-alkaline earth dimers. It is the aim to analyze the predictive power of these methods for a preliminary evaluation of potential cooling mechanisms in heteronuclear open shell systems which offer the experimentalist an electric as well as a magnetic handle for manipulation. The series of X-Sr molecules, with X = Li, Na, K and Rb, has been chosen for a direct comparison. Quantum degenerate mixtures of Rb and Sr have already been produced,B. Pasquiou, A. Bayerle, S. M. Tzanova, S. Stellmer, J. Szczepkowski, M. Parigger, R. Grimm, and F. Schreck, Phys. Rev. A, 2013, 88, 023601aking this combination very promising for the production of ultracold molecules. B. Pasquiou, A. Bayerle, S. M. Tzanova, S. Stellmer, J. Szczepkowski, M. Parigger, R. Grimm, and F. Schreck, Phys. Rev. A, 2013, 88, 023601m

Chemical bonding in alkaline earth hypermetalic oxides is of fundamental interest. Previous Ab initio studies of CaOCa predicted a centrosymmetric linear geometry for both the ¹Σ_g⁺ ground state and the low lying triplet ³Σ_u⁺ stateB. Ostojicí, P.R. Bunker, P. Schwerdtfeger, Artur Gertych, and Per Jensen, Journal of Molecular Structure 1023 (2012) 101–107. However, there have been no reports concerning the higher energy singlet and triplet states. The present work is focused on characterization of the potential energy surface (PES) of the excited ¹Σ_u⁺ state (assuming a centrosymmetric linear geometry) and obtaining predictions for the ¹Σ_u⁺←¹Σ_g⁺ vibronic transitions. We employed the multireference configuration interaction (MRCISD) method with state-averaged, full-valence complete active space self-consistent field (SA-FV-CASSCF) wavefunctions. In these calculations, the active space consisted of ten valence electrons in twelve orbitals, where all the valence electrons were correlated. Contributions of higher excitation and relativistic effects were taken into account using the Davidson correction and the Douglas-Kroll (DK) Hamiltonian, respectively. The correlation-consistent polarized weighed core-valence quadruple zeta basis set (cc-pwCVQZ-DK) was used for all three atoms. The full level of theory is abbreviated as SA-FV-CASSCF (10,12)-MRCISD-Q/cc-pwCVQZ-DK. The calculations were carried out using the MOLPRO2012 suite of programs. For the centrosymmetric linear geometry in all states, initial investigations of one-dimensional radial cuts provided equilibrium bond distances of 2.034 Å, 2.034 Å, and 1.999 Å for the ¹Σ_g⁺ , ³Σ_u⁺ , and ¹Σ_u⁺ states, respectively. The vertical excitation frequency of the ¹Σ_u⁺←¹Σ_g⁺ optical transition was calculated to occur at 14801 cm⁻¹. These predictions were followed by spectroscopic searches by Heaven et al. Indeed, rotationally resolved vibronic progressions were recorded in the vicinity of the predicted electronic band origin. Calculation of the three-dimensional PES showed that the potential minimum in the ¹Σ_u⁺ corresponds to a bent equilibrium geometry with a bond angle of 120^° (C_2v point group, where the electronic symmetry is ¹A₁). This result suggests that the Ca-O bonds in CaOCa possess covalent character in the ¹A₁ excited state and ionic character in the ¹Σ_g⁺ ground state. The current results, as well as those from ongoing studies will be presented.

---

Footnotes:

B. Ostojicí, P.R. Bunker, P. Schwerdtfeger, Artur Gertych, and Per Jensen, Journal of Molecular Structure 1023 (2012) 101–107..

An ab initio potential energy surface (PES) for NH₃ is computed using the methodology pioneered for water (Polyansky et al. J. Phys. Chem. A,117, 9633 (2013)). A multireference configuration calclulations are performed at 50000 points using quadruple and 5z basis sets to give a complete basis set (CBS) extrapolation. Relativistic and adiabatic surfaces are also computed. The points are fitted to an analytical PES. The rovibrational energy levels are computed using the program TROVE in both linearized and curvilinear coordinates. Better convergence is obtained for the higher energy levels using curvilinear coordinates: an accuracy of about 1 cm⁻¹is achieved for the levels up to 12 000 cm⁻¹. The levels up to 18 000 cm⁻¹are reproduced with the accuracy of a few cm⁻¹. These results are used to assign the visible spectrum of ¹⁴NH₃ recorded by Coy and Lehmann (J. Chem. Phys., 84, 5239 (1988)). Predicted rovibrational levels for NH₂D, NHD₂, ND₃ and ¹⁵NH₃ are given.

The O₂-SO₂ complex is believed to be a precursor to acid rain. The previously observed FTMW spectrum suggested internal motions within the complex, but their nature was not identified. Development of an effective Hamiltonian for an open-shell molecule with tunneling requires knowledge of the potential energy surface (PES) and the intrinsic reaction coordinates (IRC) for the paths between minima. A recent ab initio study reported two different nonplanar minima in the ground electronic state of O₂-SO₂. These predictions were based on geometry optimization calculations at the MP2/aug-cc-pVnZ level of theory, with n = 2 and 3. The current work is focused on a highly correlated ab initio investigation of the global PES (a 9-D problem) in the ground triplet electronic state of O₂-SO₂. Because of the high dimensionality in the complex, the PES calculations are partitioned into several two-dimensional cuts through the PES. We have so far explored only a 3-D part of the global PES to look for stable planar configurations. These calculations included geometry optimization, frequency, and single point energy calculations. Calculations were performed using UCCSD(T)/aug-cc-pV(n+D)Z,where n = 2 and 3, level of theory. We used an axis system that defines the radial and the angular van der Waals coordinates for a planar complex as R_vW, θ₁, and θ₂. The bond length (R_vW) is the distance between the center of mass of the O₂ unit and the S atom. θ₁ and θ₂ are the angles between the van der Waals bond and the O₂ internuclear axis or one of the SO bonds in the SO₂ moiety, respectively. Full geometry optimization calculations predicted a minimum of C_s symmetry in which both the O₂ and SO₂ units are tilted with respect to the van der Waals bond, and R_vW = 3.63 Å. 3-D PES surface calculations, which involve the R_vW, θ₁, and θ₂ vdW coordinates, showed that the optimized structure is the global minimum. In addition, a local minimum at R_vW = 3.9 Å, which represents a different chemical isomer, was identified. If the four oxygen atoms are labeled, each isomer is a part of four equivalent minima, and three distinguishable transition states between these various minima are identified. These results suggest that PES calculations should consider at least five dimensions. Our progress in exploring possible non-planar coordinates and IRC paths will also be presented.

Two distinct types of Quantum Monte Carlo (QMC) calculations are applied to electronic structure problems such as calculating potential energy curves and producing benchmark values for reaction barriers. First, Variational and Diffusion Monte Carlo (VMC and DMC) methods using a trial wavefunction subject to the fixed node approximation were tested using the CASINO code.[1] Next, Full Configuration Interaction Quantum Monte Carlo (FCIQMC), along with its initiator extension (i-FCIQMC) were tested using the NECI code.[2] FCIQMC seeks the FCI energy for a specific basis set. At a reduced cost, the efficient i-FCIQMC method can be applied to systems in which the standard FCIQMC approach proves to be too costly. Since all of these methods are statistical approaches, uncertainties (error-bars) are introduced for each calculated energy. This study tests the performance of the methods relative to traditional quantum chemistry for some benchmark systems. References: [1] R. J. Needs et al., J. Phys.: Condensed Matter 22, 023201 (2010). [2] G. H. Booth et al., J. Chem. Phys. 131, 054106 (2009).

For a fluorescent dye, the quantum yield characterizes the efficiency of energy transfer from the absorbed light to the emitted fluorescence. In the screening among potential families of dyes, those with higher quantum yields are expected to have more advantages. From the perspective of theoreticians, an efficient prediction of the quantum yield using a universal excited state electronic structure theory is in demand but still challenging. The most representative examples for such excited state theory include time-dependent density functional theory (TDDFT) and restricted open-shell Kohn-Sham (ROKS)T. Kowalczyk, T. Tsuchimochi, L. Top, P.-T. Chen, and T. Van Voorhis, J. Chem. Phys., 138, 164101 (2013). In the present study, we explore the possibility of predicting the quantum yields for conventional and new families of organic dyes using a combination of TDDFT and ROKS. We focus on radiative (k_r) and nonradiative (k_nr) rates for the decay of the first singlet excited state (S₁) into the ground state (S₀) in accordance with Kasha's rule.M. Kasha, Discuss. Faraday Soc., 9, 14 (1950).For each dye compound, k_r is calculated with the S₁−S₀ energy gap and transition dipole moment obtained using ROKS and TDDFT respectively at the relaxed S₁ geometry. Our predicted k_r agrees well with the experimental value, so long as the order of energy levels is correctly predicted. Evaluation of k_nr is less straightforward as multiple processes are involved. Our study focuses on the S₁−T₁ intersystem crossing (ISC) and the S₁−S₀ internal conversion (IC): we investigate the properties that allow us to model the k_nr value using a Marcus-like expression, such as the Stokes shift, the reorganization energy, and the S₁−T₁ and S₁−S₀ energy gaps. Taking these factors into consideration, we compare our results with those obtained using the actual Marcus theory and provide explanation for discrepancy.

---

Footnotes:

T. Kowalczyk, T. Tsuchimochi, L. Top, P.-T. Chen, and T. Van Voorhis, J. Chem. Phys., 138, 164101 (2013).. M. Kasha, Discuss. Faraday Soc., 9, 14 (1950).

Rovibrational transitions associated with tunneling states in the water bending vibration in OC-H₂O and other available spectroscopic data are included in generation of 6-D morphed potentials of the complex. Six-dimension ab initio interaction potentials are initially calculated for the complex to provided the initial functions for the potential morphing. The available spectroscopic data is then used to fit and generate 6-D morphed potentials. Previous prediction of the D₀ of the complex will be incorporated in the analysis. Finally, intermolecular frequencies of the complex will be predicted using the 6-D morphed potentials involving the CO stretching and the H₂O bending vibrations.