Apostolos Kalemos

An accurate description of the ground and excited states of SiH

With the high accuracy afforded by the sextuple correlation consistent basis set of Dunning, we have calculated energy levels, dissociation energies, equilibrium distances, and other spectroscopic constants for eleven valence and four Rydberg states of the CH radical. Comparisons with experimental and previous theoretical results are made for each state that has been treated. An understanding of their binding is attempted by means of simple valence bond–Lewis diagrams.

The Sc2 dimer revisited

Thirty two states of the homonuclear neutral diatomic Sc(2) molecule have been studied by multireference methods using basis sets of quadruple quality. For all 30 states resulting from the ground state Sc atoms, Sc((2)D(g))+Sc((2)D(g)), and two out of 80, X (5)Sigma(u) (-) and 1 (3)Sigma(u) (-), issued from the first excited channel Sc((2)D(g))+Sc(a (4)F(g)), we have constructed full potential energy curves and extracted the standard spectroscopic parameters. With the exception of X (5)Sigma(u) (-) and 1 (3)Sigma(u) (-) which are covalently bound, the 30 states related to the ground state Sc atoms are of van der Waals nature with interaction energies of 3-5 kcal/mol at distances of 7-7.5 bohr. For the X (5)Sigma(u) (-) state the proposed D(e) value is 48 kcal/mol, with respect to the adiabatic fragments and with the 1 (3)Sigma(u) (-) state just 380 cm(-1) above it.

The electronic structure of vanadium carbide, VC

Within an energy range of 2.4 eV, we have explored 29 of the 36 states of the diatomic molecule VC that arise from the atoms in their ground state, V(4s23d3;4F)+C(2s2 2p2;3P). We use multireference methods with large atomic natural orbital basis sets. The ground state is of 2Delta symmetry with the first two excited states, 4Delta and 2Sigma+, located 4.2 and 7.0 kcal/mol above the X state. All the states examined in this work are relatively strongly bound and show significant charge transfer from V to C. The binding energy of the X 2Delta state is estimated to be 95.3 kcal/mol in good agreement with the experimental value.

On symmetry breaking in BNB: Real or artifactual?

The ground state of the linear BNB molecule has been examined with multireference-based ab initio methods coupled with quantitative basis sets. Previous computational studies on BNB, even those using highly correlated single reference-based methods, e.g., the CCSD(T) and BDT methods, suggested that the two BN bond lengths were unequal. In this paper, the BN(X 3Pi) + B(2Pu) potential energy curve is constructed using a state-averaged multireference-based correlated method (SA-CASSCF+PT2). The four lowest states of BN were included in the state averaging procedure. These calculations reveal no symmetry breaking along the antisymmetric stretching mode of the molecule.

First principles investigation of chromium carbide, CrC

We have investigated the electronic structure of 14 states of the experimentally unknown diatomic molecule chromium carbide, CrC, using standard multireference configuration interaction methods and high quality basis sets. We report potential curves, binding energies, and a number of spectroscopic parameters. The ground state of CrC, X 3Sigma-, displays triple-bond character with a binding energy of D(e)=89 kcal/mol and an internuclear separation of r(e)=1.63 A. The first excited state (1 5Sigma-) lies 9.2 kcal/mol higher. All the states studied are fairly ionic, featuring an electron transfer of 0.3-0.5e- from the metal atom to the carbon atom.

The electronic structure of Ti2 and Ti2

The Ti(2) and Ti(2)(+) molecular systems have been studied through multireference variational and single reference coupled-cluster methods coupled with large basis sets. Potential energy curves have been constructed for 30 (Ti(2)) and 2 (Ti(2)(+)) states and the usual spectroscopic parameters have been extracted. The main feature of the potential curves is the existence of van der Waals minima (Ti(2)) around 7 bohr irrespective of the molecular symmetry, and 4s(2)-4s(1) interactions (Ti(2)(+)) around 6 bohr. Numerous avoided crossings lead to stronger covalent bonds emanating from 4s(1)-4s(1) atomic distributions. The X-state of the neutral species is formally a (3)Δ(g) state with the first excited state lying within 1 kcal/mol. The removal of the symmetry defining e(-) leads to the X(2)Σ(g)(+) state of Ti(2)(+).

An ab initio study of the E 3Πg state of the iodine molecule.

The E (3)Π(g) state of the iodine molecule is studied by ab initio multireference methods coupled with effective core potentials and large basis sets. Two potential minima are found, a global featuring an ion-pair character, and a local presenting a purely Rydberg nature. Four avoided crossings along the dissociation coordinate attribute an interesting topology to its potential energy curve, and their effect on the vibrational levels of I(2) is discussed.

Ab initio study of the electronic structure of manganese carbide

We report electronic structure calculations on 13 states of the experimentally unknown manganese carbide (MnC) using standard multireference configuration interaction (MRCI) methods coupled with high quality basis sets. For all states considered we have constructed full potential energy curves and calculated zero point energies. The X state, correlating to ground state atoms, is of 4sigma- symmetry featuring three bonds, with a recommended dissociation energy of D0 = 70.0 kcal/mol and r(e) = 1.640 angstroms. The first and second excited states, which also correlate to ground state atoms, are of 6sigma- and 8sigma- symmetry, respectively, and lie 17.7 and 28.2 kcal/mol above the X state at the MRCI level of theory.

The nature of the chemical bond in Be2+, Be2, Be2−, and Be3

The present study focuses on the Be2+, Be2, Be2-, and Be3 species with the aim to unveil their bonding pattern. The ground states of the above molecules are examined mainly through multi reference configuration interaction methods using an aug-cc-pVQZ basis set. Previous work is scrutinized with an eye to the instability problems inherent in various Hartree-Fock schemes. Under this point of view, we rationalize previous findings and put them in a unified context of what and why should be done in similar hard to crack cases. In all the above species, the covalently bound ground states are formed just because the Be atom is found in its first excited 3P(2s12p1) state.

Symmetry breaking in a nutshell: The odyssey of a pseudo problem in molecular physics. The X̃Σu+2 BNB case revisited

The X̃(2)Σu (+) BNB state considered to be of symmetry broken (SB) character has been studied by high level multireference variational and full configuration interaction methods. We discuss in great detail the roots of the so-called SB problem and we offer an in depth analysis of the unsuspected reasons behind the double minimum topology found in practically all previous theoretical investigations. We argue that the true reason of failure to recover a D∞h equilibrium geometry lies in the lack of the correct permutational symmetry of the wavefunctions employed and is by no means a real effect.