Using computed infrared intensities for the reduction of vibrational configuration interaction bases

International audienceThe Adaptive Vibrational Configuration Interaction (A-VCI) algorithm is an iterative process that computes the anharmonic spectrum of a molecule using nested bases to discretize the Hamiltonian operator. For large molecular systems, the size of the discretization space and the computation time quickly become prohibitive. It is therefore necessary to develop new methods to further limit the number of basis functions. Most of the time, the interpretation of an experimental infrared spectrum does not require the calculation of all eigenvalues but only those corresponding to vibrational states with significant intensity. In this paper, a technique that uses infrared intensities is introduced to select a subset of eigenvalues to be precisely calculated. Thus, we build smaller nested bases and reduce both the memory footprint and the computational time. We validate the advantages of this new approach on a well-studied 7-atom molecular system (C2H4O), and we apply it on a larger 10-atom molecule (C4H4N2)

Using computed infrared intensities for fast computation of vibrational spectra

The Adaptive Vibrational Configuration Interaction (A-VCI) algorithm is an iterative pro-cess able to compute the spectrum of an Hamiltonian operator, using a discretization basisas small as possible. In this work, we show how this algorithm can handle more sophis-ticated operators, which ro-vibrational Coriolis coupling terms. In order to overcome theincrease of computing and storage resources needed due to this enrichment, the InfraRed(IR) intensities are computed and used as a criterion to select only the eigenstates corre-sponding to IR active vibrational states. The benefits of this new approach are presentedfor a few well studied molecular systems (H2O, H2CO, CH2NH, CH3CN, C2H4O), and itis ultimately applied to a 10-atom molecule (C4H4N2

A-VCI: une méthode flexible pour calculer rapidement des spectres vibrationnels

The Adaptive Vibrational Configuration Interaction (A-VCI) algorithm has been introduced as a new method to efficiently reduce the dimension of the set of basis functions used in a Vibrational Configuration Interaction (VCI) process. It is based on the construction of nested basis for the discretization of the Hamiltonian operator according to a theoretical criterion that ensures the convergence of the method. The purpose of this paper is to study the properties and outline the performance details of the main steps of the algorithm. New parameters have been incorporated to increase flexibility, and their influence have been thoroughly investigated. The robustness and reliability of the method are demonstrated for the computation of the vibrational spectrum up to $3000$ cm$^{-1}$ of a widely studied $6$-atom molecule (acetonitrile). Our results are compared to the most accurate up to date computation, and we also give a new reference calculation for future work on this system. The algorithm has also been applied to a more challenging $7$-atom molecule (ethylene oxide). The computed spectrum up to $3200$ cm$^{-1}$ is the most accurate computation that exists today on such a system.L'algorithme adaptatif d'interaction de configuration vibrationnelle (A-VCI) a été introduit comme une nouvelle méthode pour réduire efficacement la dimension de l'ensemble des fonctions de base utilisées dans un processus d'interaction de configuration vibrationnelle (VCI). Il est basé sur la construction de bases emboîtées pour la discrétisation de l'opérateur Hamiltonien selon un critère théorique qui assure la convergence de la méthode. Cet article présente les propriétés de la méthode et décrit les détails des principales étapes de l'algorithme. De nouveaux paramètres sont introduits pour accroître les potentialités de la méthode et leurs influences sont étudiées. La robustesse et la fiabilité de la méthode sont démontrées pour le calcul du spectre vibrationnel jusqu'à $3000$ cm $^ {- 1}$ d'une molécule à 6 atomes (acétonitrile). Nos résultats sont comparés au calcul le plus précis à jour, et nous donnons également un nouveau calcul de référence pour les travaux futurs sur ce système. L'algorithme a également été appliqué à un système plus difficile l'oxyde d'éthylène qui comporte 7 atomes. Le spectre calculé jusqu'à $3200$ cm $^{- 1}$ est le calcul le plus précis qui existe aujourd'hui sur un tel système

Efficient basis selection for the computation of vibrational spectrum

International audienceWe propose here an efficient method to define an approximation subspace to compute the first vibrational frequencies of the molecular Hamiltonian which are those of interest in the experimental results

Optimisation of the variational method for vibrational Hamiltonian eigenvalues computation

International audienceWe propose here an efficient method to define a representative approximation subspace to compute the first eigenvalues of the vibrational Hamiltonian which are those of interest in the experimental results

Adaptive vibrational configuration interaction (A-VCI): a posteriori error estimation to efficiently compute anharmonic IR spectra

International audienceA new variational algorithm called adaptive vibrational configuration interaction (A-VCI) intended for the resolution of the vibrational Schrödinger equation was developed. The main advantage of this approach is to efficiently reduce the dimension of the active space generated into the configuration interaction (CI) process. Here, we assume that the Hamiltonian writes as a sum of products of operators. This adaptive algorithm was developed with the use of three correlated conditions i.e. a suitable starting space ; a criterion for convergence, and a procedure to expand the approximate space. The velocity of the algorithm was increased with the use of a posteriori error estimator (residue) to select the most relevant direction to increase the space. Two examples have been selected for benchmark. In the case of H 2 CO, we mainly study the performance of A-VCI algorithm: comparison with the variation-perturbation method, choice of the initial space, residual contributions. For CH 3 CN, we compare the A-VCI results with a computed reference spectrum using the same potential energy surface and for an active space reduced by about 90 %

Concentration dependent refractive index of a binary mixture at high pressure

In the present work binary mixtures of varying concentrations of two miscible hydrocarbons, 1,2,3,4-tetrahydronaphtalene (THN) and n-dodecane (C12), are subjected to increasing pressure up to 50 MPa in order to investigate the dependence of the so-called concentration contrast factor (CF), i.e., (∂n/∂c)p, T, on pressure level. The refractive index is measured by means of a Mach-Zehnder interferometer. The setup and experimental procedure are validated with different pure fluids in the same pressure range. The refractive index of the THN-C12 mixture is found to vary both over pressure and concentration, and the concentration CF is found to exponentially decrease as the pressure is increased. The measured values of the refractive index and the concentration CFs are compared with values obtained by two different theoretical predictions, the well-known Lorentz-Lorenz formula and an alternative one proposed by Looyenga. While the measured refractive indices agree very well with predictions given by Looyenga, the measured concentration CFs show deviations from the latter of the order of 6% and more than the double from the Lorentz-Lorenz prediction

Linear and nonlinear optical properties of a series of Ni-dithiolene derivatives

Some linear and nonlinear optical (NLO) properties of Ni(SCH)4 and several of its derivatives have been computed by employing a series of basis sets and a hierarchy of methods (e.g., HF, DFT, coupled cluster, and multiconfigurational techniques). The electronic structure of Ni(SCH)4 has been also analyzed by using CASSCF/CASPT2, ab initio valence bond, and DFT methods. In particular we discuss how the diradicaloid character (DC) of Ni(SCH)4 significantly affects its NLO properties. The quasidegeneracy of the two lowest-energy singlet states 1 mathg and 1 math1u, the clear DC nature of the former, and the very large number of low-lying states enhance the NLO properties values. These particular features are used to interpret the NLO properties of Ni(SCH)4. The DC of the considered derivatives has been estimated and correlated with the NLO properties. CASVB computations have shown that the structures with Ni(II) are the dominant ones, while those with Ni(0) and Ni(IV) have negligible weight. The weights of the four diradical structures were discussed in connection with the weight of the structures, where all the electrons are paired. Comparative discussion of the properties of Ni(SCH)4 with those of tetrathia fulvalene demonstrates the very large effect of Ni on the properties of the Ni-dithiolene derivatives. A similar remarkable effect on the NLO properties is produced by one or two methyl or C3S groups. The considered Ni-dithiolene derivatives have exceptionally large NLO properties. This feature in connection with their other physical properties makes them ideal candidates for photonic [email protected]

Carbenic nitrile imines: Properties and reactivity

Structures and properties of nitrile imines were investigated computationally at B3LYP and CCSD(T) levels. Whereas NBO analysis at the B3LYP DFT level invariably predicts a propargylic electronic structure, CCSD(T) calculations permit a clear distinction between propargylic, allenic, and carbenic structures. Nitrile imines with strong IR absorptions above ca. 2150 cm-1 have propargylic structures with a CN triple bond (RCNNSiMe 3 and R2BCNNBR2), and those with IR absorptions below ca. 2150 cm-1 are allenic (HCNNH, PhCNNH, and HCNNPh). Nitrile imines lacking significant cumulenic IR absorptions at 1900-2200 cm -1 are carbenic (R-(C:)-N=N-R′). Electronegative but lone pair-donating groups NR2, OR, and F stabilize the carbenic form of nitrile imines in the same way they stabilize "normal" singlet carbenes, including N-heterocyclic carbenes. NBO analyses at the CCSD(T) level confirm the classification into propargylic, allenic, and carbenic reactivity types. Carbenic nitrile imines are predicted to form azoketenes 21 with CO, to form [2+2] and [2+4] cycloadducts and borane adducts, and to cyclize to 1H-diazirenes of the type 24 in mildly exothermic reactions with activation energies in the range 29-38 kcal/mol. Such reactions will be readily accessible photochemically and thermally, e.g., under the conditions of matrix photolysis and flash vacuum thermolysis