Roberto Linguerri

Quantum chemistry using the density matrix renormalization group

A new implementation of the density matrix renormalization group is presented for ab initio quantum chemistry. Test computations have been performed of the dissociation energies of the diatomics Be2, N2, HF. A preliminary calculation on the Cr2 molecule provides a new variational upper bound to the ground state energy.

Quantum chemistry using the density matrix renormalization group II

We have compared different strategies for ab initio quantum chemistry density matrix renormalization group treatments. The two starting orbital blocks include all valence and active orbitals of the reference complete active space self consistent field wave function. To generate the remaining blocks, we propose following the order of the contributions to the correlation energy: a posteriori using approximate occupation numbers or a priori, using a Moller–Plesset type of arguments, by explicit evaluation of second-order interactions. We have compared two different schemes for orbital localization to identify the important and less important orbital interactions and simplify the generation of the orbital blocks. To truncate the expansion we have compared two approaches, keeping constant the number m of components or the threshold λ to fix the residue of the expansion at each step. The extrapolation of the energies is found to provide accurate estimates of the full configuration interaction energy, making the...

Characterization of the MgO2+ dication in the gas phase: electronic states, spectroscopy and atmospheric implications

Franzreb and Williams at Arizona State University detected recently the MgO(2+) molecular species in the gas phase. Here we report a very detailed theoretical investigation of the low-lying electronic states of this dication including their potentials, spin-orbit, rotational and radial couplings. Our results show that the potential energy curves of the dicationic electronic states have deep potential wells. This confirms that this dication does exist in the gas phase; it is a thermodynamically stable molecule in its ground state, and it has several excited long-lived metastable states. The potential energy curves are used then to predict a set of spectroscopic parameters for the bound states of MgO(2+). We have also incorporated these potentials, rotational and radial couplings in dynamical calculations to derive the cross sections for the charge transfer Mg(2+) + O → Mg(+) + O(+) reaction in the 1-10(3) eV collision energy domain via formation-decomposition of the MgO(2+) dication. Our work shows the role of MgO(2+) in the Earth ionosphere and more generally in atmospheric processes in solar planets, where this reaction efficiently participates in the predominance of Mg(+) cations in these media compared to Mg and Mg(2+).

Systematic theoretical studies of the interaction of 1,4-diazabicyclo [2.2.2]octane (DABCO) with rare gases

We investigate the intermolecular 1,4-diazabicyclo [2.2.2] octane (DABCO) interaction potential with He, Ne, Ar, and Kr rare gases (Rg) by means of post Hartree-Fock and the newly implemented explicitly correlated coupled cluster approaches in connection with several basis sets. After benchmarking computations, we show that the inclusion of diffuse atomic orbitals is mandatory for the accurate description of structures, energetics, and spectroscopic properties of DABCO-Rg van der Waals clusters and that the (R)MP2∕aug-cc-pVXZ (X = D, T) level is accurate enough for that purposes. For the neutral and ionic DABCO-Rg complexes, we characterized the low energy stationary points on the ground state potential. Most of the computed structures show a distortion along the low frequency mode of the van der Waals complex. Tunneling through this potential barrier leads to a splitting of the ground vibrational levels of several cm(-1). Our results served to reassign the available experimental spectra for DABCO(0,+1)-Ar and DABCO(0,+1)-Kr.

Simplest N-Sulfonylamine HNSO2.

The simplest N-sulfonylamine HNSO2 has been generated in the gas phase through flash vacuum pyrolysis of methoxysulfonyl azide CH3OS(O)2N3. Its identification was accomplished by combining matrix-isolation IR spectroscopy and quantum chemical calculations. Both experimental and theoretical evidence suggest a stepwise decomposition of the azide via the methoxysulfonyl nitrene CH3OS(O)2N, observed in the 193 nm laser photolysis of the azide, with concerted fragmentation into CH2O and HNSO2. Upon the 193 nm laser irradiation, HNSO2 isomerizes into the novel N-hydroxysulfinylamine HONSO.

Solvation effects and stabilization of multicharged ions: a case study of ArmBeOq+ complexes

Using first-principle methods, we characterized Ar(m)BeO(q+) (0 ≤ m ≤ 3 and 0 ≤ q ≤ 2) and BeO(q+) (3 ≤ q ≤ 5) multicharged ions (MCIs). This includes their structural parameters, relative stabilities and vibrational wavenumbers. Our calculations confirm the existence of the ArBeO complex. In addition, we found (meta-)stable neutral and ionic complexes such as ArBeO(+), ArBeO(2+), Ar(2)BeO, Ar(2)BeO(+), Ar(2)BeO(2+), Ar(3)BeO, Ar(3)BeO(+) and Ar(3)BeO(2+). The analysis of the structural parameters and of the electron density differences shows that a strong perturbation in the electronic structure of the BeO(q+) (q = 1,2) moiety occurs upon complexation, resulting in a major increase in covalency of these multicharged ions (MCIs). The consequences of solvation of MCIs in argon matrices are pointed out. More generally, the effects on the spectroscopy of MCIs trapped on cold matrices are discussed.

Theoretical spectroscopic investigations of HNSq and HSNq (q=0,+1,-1) in the gas phase

We performed accurate ab initio investigations of the geometric parameters and the vibrational structure of neutral HNS/HSN triatomics and their singly charged anions and cations. We used standard and explicitly correlated coupled cluster approaches in connection with large basis sets. At the highest levels of description, we show that results nicely approach those obtained at the complete basis set limit. Moreover, we generated the three-dimensional potential energy surfaces (3D PESs) for these molecular entities at the coupled cluster level with singles and doubles and a perturbative treatment of triple excitations, along with a basis set of augmented quintuple-zeta quality (aug-cc-pV5Z). A full set of spectroscopic constants are deduced from these potentials by applying perturbation theory. In addition, these 3D PESs are incorporated into variational treatment of the nuclear motions. The pattern of the lowest vibrational levels and corresponding wavefunctions, up to around 4000 cm(-1) above the corresponding potential energy minimum, is presented for the first time.

Vibrations in the B4 rhombic structure

A double minimum six-dimensional potential energy surface (PES) is determined in symmetry coordinates for the most stable rhombic (D2h) B4 isomer in its 1Ag electronic ground state by fitting to energies calculated ab initio. The PES exhibits a barrier to the D4h square structure of 255 cm(-1). The vibrational levels (J=0) are calculated variationally using an approach which involves the Watson kinetic energy operator expressed in normal coordinates. The pattern of about 65 vibrational levels up to 1600 cm(-1) for all stable isotopomers is analyzed. Analogous to the inversion in ammonia-like molecules, the rhombus rearrangements lead to splittings of the vibrational levels. In B4 it is the B1g (D4h) mode which distorts the square molecule to its planar rhombic form. The anharmonic fundamental vibrational transitions of 11B4 are calculated to be (splittings in parentheses): G(0)=2352(22) cm(-1), nu1(A1g)=1136(24) cm(-1), nu2(B1g)=209(144) cm(-1), nu3(B2g)=1198(19) cm(-1), nu4(B2u)=271(24) cm(-1), and nu5(Eu)=1030(166) cm(-1) (D4h notation). Their variations in all stable isotopomers were investigated. Due to the presence of strong anharmonic resonances between the B1g in-plane distortion and the B2u out-of-plane bending modes, the higher overtones and combination levels are difficult to assign unequivocally.

On the role of the simplest S-nitrosothiol, HSNO, in atmospheric and biological processes

Using state-of-the-art theoretical methods, we investigate the lowest electronic states of singlet and triplet spin multiplicities of HSNO. These computations are done using configuration interaction ab initio methods and the aug-cc-pV5Z basis set. One-dimensional cuts of the six-dimensional potential energy surfaces of these electronic states along the HS, SN stretches and HSN, SNO bending and torsion coordinates are calculated. Several avoided crossings and conical intersections are found. We computed also radiative lifetimes and spin-orbit couplings of these electronic states. Our work shows that the dynamics on these excited states is very complex, and suggest that multi-step mechanisms will populate the ground state via radiationless processes or lead to predissociation or intramolecular isomerization. For instance, these potentials are used to propose mechanisms for the IR, Vis, and UV light-induced cis-trans interconversions of HSNO and reactivity towards HS + NO and H + SNO products. Our findings are in good agreement with previous experimental studies on the photochemistry of HSNO. The atmospheric implication of HSNO is also discussed.

Structural and spectroscopic study of the van der Waals complex of CO with HCO+ and the isoelectronic complex of CS with HCS+

This work reports the results of high level ab initio calculations of the OC-HCO(+) complex and the SC-HCS(+) complex and their hydrogen migration transition states. Geometry optimizations are performed at the CCSD(T)/aug-cc-pV5Z level of theory. Subsequent frequency calculations are carried out at the CCSD(T)/aug-cc-pVQZ level of theory. Additional geometry optimizations and harmonic frequency calculations for all the species involved in this study have been done with the explicitly correlated CCSD(T)-F12 method with the aug-cc-pVTZ and VTZ-F12 basis set. The geometries, rotational constants, harmonic vibrational frequencies, and energetics of the species involved in the complex are reported. These methods result in accurate computational predictions that have mean deviations for bond lengths, rotational constants, and vibrational frequencies of 0.001 Å, 163 MHz, and 46 cm(-1), respectively. These results provide essential spectroscopic properties for the complexes that can facilitate both laboratory and interstellar observations, and they also provide a comparison between oxygen and sulfur complex observability based on thermodynamic stability.