Calculation of IR frequencies and intensities in electrical and mechanical anharmonicity approximations: Application to small water clusters (original) (raw)
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The Journal of Chemical Physics, 2010
It is well known that the convergence of harmonic frequencies with respect to the basis set size in traditional correlated calculations is slow. We now report that the convergence of cubic and quartic force constants in traditional CCSD͑T͒ calculations on H 2 O with Dunning's cc-pVXZ family of basis sets is also frustratingly slow. As an alternative, we explore the performance of R12-based explicitly correlated methods at the CCSD͑T͒ level. Excellent convergence of harmonic frequencies and cubic force constants is provided by these explicitly correlated methods with R12-suited basis irrespective of the used standard approximation and/or the correlation factor. The Slater type geminal, however, outperforms the linear r 12 for quartic force constants and vibrational anharmonicity constants. The converged force constants from explicitly correlated CCSD͑T͒ calculations succeed in reproducing the fundamental frequencies of water molecule with spectroscopic accuracy after corrections for post-CCSD͑T͒ effects are made.
Structure and vibrational spectra of small water clusters from first principles simulations
The Journal of Chemical Physics, 2010
The structure and vibrational spectra of ͑H 2 O͒ n ͑n =2-5͒ clusters have been studied based on first-principles molecular dynamics simulations. Trends of the cluster structures with the cluster size show that water molecules in cluster are bound more tightly. The vibrational spectra as a function of cluster size and temperature are obtained using Fourier transformation of the velocity autocorrelation function. Results of the clusters in ground state show that when the cluster size increases, the librational peaks shift to blue and the bonded intramolecular OH stretching bands shift to red due to the clusterization and hydrogen-bond strengthening. Meanwhile, there are no significant shifts in the intramolecular bending and free OH stretching modes, indicating that the free hydrogen atoms are insensitive to the local bonding environment. The temperature-dependent vibrational spectra, which exhibit similar behaviors from the dimer to pentamer, show that there are significant broadenings of the spectra with temperature caused by thermal motions. Moreover, different bands shift to different directions, where librational bands shift to red while bonded OH stretching bands shift to blue, although the blueshifts are quite small for the dimer and trimer.
Small Clusters of Water Molecules Using Density Functional Theory
The Journal of Physical Chemistry, 1996
The geometries, interaction energies, and harmonic vibrational frequencies of water clusters (with up to 8 molecules) have been studied using density functional theory (DFT) at the gradient corrected level. The water monomer and water dimer calculations have been used as benchmarks to investigate different choices for basis sets and density functionals. Our results for larger clusters agree with both available high-level ab initio calculations and experimental information. The calculations of the vibrational frequencies and IR absorption intensities for the larger clusters, for which no other reliable quantum-chemical calculation is available, are presented to facilitate the frequency assignment of experimental spectra.
On the Contribution of Vibrational Anharmonicity to the Binding Energies of Water Clusters
The Journal of Physical Chemistry A, 2005
The second-order vibrational perturbation theory method has been used together with the B3LYP and MP2 electronic structure methods to investigate the effects of anharmonicity on the vibrational zero-point energy (ZPE) contributions to the binding energies of (H 2 O) n , n) 2-6, clusters. For the low-lying isomers of (H 2 O) 6 , the anharmonicity correction to the binding energy is calculated to range from-248 to-355 cm-1. It is also demonstrated that although high-order electron correlation effects are important for the individual vibrational frequencies, they are relatively unimportant for the net ZPE contributions to the binding energies of water clusters.
The Journal of Chemical Physics, 2011
Infrared photodissociation spectroscopy is reported for mass-selected H + (H 2 O) n complexes and their deuterated analogues with and without argon "tagging." H + (H 2 O) n Ar m and D + (D 2 O) n Ar m complexes are studied in the O-H (O-D) stretching region for clusters in the small size range (n ) 2-5). Upon infrared excitation, these clusters fragment by the loss of either argon atoms or one or more intact water molecules. Their excitation spectra show distinct bands in the region of the symmetric and asymmetric stretches of water and in the hydrogen bonding region. Experimental studies are complemented by computational work that explores the isomeric structures, their energetics and vibrational spectra. The addition of an argon atom is essential to obtain photodissociation for the n ) 2-3 complexes, and specific inclusion of the argon in calculations is necessary to reproduce the measured spectra. For n ) 3-5, spectra are obtained both with and without argon. The added argon atom allows selection of a subset of colder clusters and it increases the photodissociation yield. Although most of these clusters have more than one possible isomeric structure, the spectra measured correspond to a single isomer that is computed to be the most stable. Deuteration in these small cluster sizes leads to expected lowering of frequencies, but the spectra indicate the presence of the same single moststable isomer for each cluster size.
Infrared Predissociation Spectroscopy of Large Water Clusters: A Unique Probe of Cluster Surfaces
The Journal of Physical Chemistry A
The vibrational OH-stretch spectra of large water clusters were measured by photofragment spectroscopy after the absorption of pulsed tunable infrared radiation in the frequency range from 3000 to 3800 cm-1. The mean size of the clusters from 〈n〉) 20 to 1960 was measured by threshold photoionization of the water clusters doped with sodium atoms. The largest abundance of the fragments was that of water hexamers. The fragment intensities are measured for different excitation energies and different cluster temperatures as function of the cluster size. For the selected sizes 〈n〉) 48, 111, 631, and 1960 complete OH-stretch spectra have been measured. The comparison with calculations revealed that the method is mainly sensitive to the outer cluster surface which has for all sizes an amorphous structure dominated by 3-coordinated and to a lesser extent also by 4-coordinated molecules. The intensity of the hexamer fragments goes through a maximum at n) 70 and drops to n) 300 where it levels off with a different slope. This behavior is attributed to the number of available connected 3-coordinated water molecules and the influence of the emerging 4-coordinated molecules in these clusters.
Journal of Physical Chemistry A, 2008
We have calculated frequencies and intensities of fundamental and overtone vibrational transitions in water and water dimer with use of different vibrational methods. We have compared results obtained with correlationcorrected vibrational self-consistent-field theory and vibrational second-order perturbation theory both using normal modes and finally with a harmonically coupled anharmonic oscillator local mode model including OH-stretching and HOH-bending local modes. The coupled cluster with singles, doubles, and perturbative triples ab initio method with augmented correlation-consistent triple-Dunning and atomic natural orbital basis sets has been used to obtain the necessary potential energy and dipole moment surfaces. We identify the strengths and weaknesses of these different vibrational approaches and compare our results to the available experimental results.
Infrared spectroscopy of negatively charged water clusters: Evidence for a linear network
Journal of Chemical Physics, 1999
We report autodetachment spectra of the mass-selected, anionic water clusters, (H 2 O) n Ϫ , nϭ2, 3, 5-9, 11 in the OH stretching region ͑3000-4000 cm Ϫ1 ͒, and interpret the spectra with the aid of ab initio calculations. For nу5, the spectra are structured and are generally dominated by an intense doublet, split by about 100 cm Ϫ1 , which gradually shifts toward lower energy with increasing cluster size. This behavior indicates that the nϭ5-11 clusters share a common structural motif. The strong bands appear in the frequency region usually associated with single-donor vibrations of water molecules embedded in extended networks, and theoretical calculations indicate that the observed spectra are consistent with linear ''chainlike'' (H 2 O) n Ϫ species. We test this assignment by recording the spectral pattern of the cooled ͑argon solvated͒ HDO•(D 2 O) 5 Ϫ isotopomer over the entire OH stretching frequency range.
The Journal of Chemical Physics, 2019
The vibrational contributions to the average polarizability (ᾱ), to the second harmonic scattering (SHS) first hyperpolarizability (βSHS), and depolarization ratio (DRSHS), as well as to the third harmonic scattering (THS) second hyperpolarizability (γ THS) and depolarization ratio (DR THS), have been evaluated for the water molecule using the Bishop and Kirtman perturbative theory approach, in combination with finite differentiation techniques to evaluate the higher-order derivatives. From a hierarchy of coupled cluster techniques and extended atomic basis sets, the CCSD/d-aug-cc-pVTZ level has been selected to assess the importance of the zero-point vibrational average (ZPVA) contributions and of the pure vibrational contributions with respect to their electronic counterparts. This is the first investigation demonstrating electronic and vibrational SHS, and THS responses can be computed for small molecules, with the perspective of performing comparisons with recent experimental data [