Aristides Mavridis

Theoretical investigation of iron carbide, FeC

Employing multireference variational methods (MRCI), we have constructed full potential-energy curves for the ground state (X 3Δ) and forty excited states of the diatomic carbide, FeC. For all states we report potential-energy curves, bond lengths, dissociation energies, dipole moments, and certain spectroscopic constants, trying at the same time to get some insight on the bonding mechanisms with the help of Mulliken populations and valence-bond–Lewis diagrams. For the X 3Δ state at the MRCI level of theory, we obtain a dissociation energy De=86.7 kcal/mol at a bond length re=1.581 A. These values compare favorably to the corresponding experimental ones, De=91.2±7 (upper limit) kcal/mol and re=1.5924 A. The first excited state (1Δ) is predicted to be 9.7 kcal/mol above the X-state as compared to an experimental value of 9.786 kcal/mol.

A reinvestigation of tolane

Cl4Hl0, monoclinic, P2Ja, a = 12.778 (2), b = 5.764 (1), c = 15.508 (4) A, f l = 113.39 (2 ) ° ,Z = 4, R w = 0.036 for 1305 counter reflections. The asymmetric unit consists of two crystallographically independent half molecules with similar bond distances and angles. The molecules are planar. Semi-empirical CNDO/2 calculations predict a quasi free rotation of the phenyl ring around the C C single bond with a rotation barrier of 0.65 kcal molIntroduction. The structure of 1,2-diphenylacetylene was examined by Robertson & Woodward (1938). Their experimental data were limited to 106 reflections in the hOl plane, and the results are not very accurate. We have re-examined tolane counter data, to obtain better structural parameters. Tolane was synthesized by a standard method (Fieser & Fieser, 1967). Crystals were obtained by cooling (24 h) a solution in methanol. A suitable crystal, 0.6 x 0.6 x 0.4 mm, covered with glue to prevent sublimation, was used for data collection. The absences hOl: h = 2n, and 0k0: k = 2n and the monoclinic symmetry verified the previously reported space group (Robertson & Woodward, 1938). However, during the structure determination it was realized that the present unit cell (I) is related to that of Robertson & Woodward (II) by: a(I) = a(II), b(I) = -b(II ) , e(I) = [a ( I I ) + e(II)]. The density measured by flotation in CaC12 solution is 1.136 g cm -3 at 23°C and the calculated density based on four molecules per unit cell is 1.129 g cm -3 at 25°C. Diffraction data were collected at an average temperature of 24 .5°C with a Picker FACS-I automatic diffractometer using Zr-filtered Mo K¢~ radiation (/t = 0-69 cm-~). The lattice parameters were obtained by least-squares refinement of the 20, o~, g and tp angles of 12 reflections in the range 35 < 20 < 39 °, for which the ttl~t2 doublet of Mo K~t radiation was resolved. Intensities of reflections were measured by the 0-20 scan method with a scan speed of 1 °(20) min -~ and a 10 s background count at the start and end of each scan. The intensities of three standard reflections (232, 22(], 218) measured every 50 reflections were used to monitor X-ray damage and alignment of the crystal. Of 2062 unique reflections in the range 20 < 50 °, 209 were systematically absent, and 548 with I < a(I) were considered unobserved. The intensities were corrected for Lorentz and polarization factors, and for decay based on two different slopes: up to sin 0/2 = 0.54 A -I a maximum linear correction of 9% was applied and to the rest of the data (0.54 < sin 0/2 < 0.60 A -1) a maximum of 20%. Standard deviations for the structure factors were obtained from counting statistics (Wei & Ward, 1976). No corrections for absorption or extinction were made. Attenuation filters were used to keep the intensities within the linearity range of the counter. Approximate scale and isotropic temperature factors were determined by Wilsons (1942) method. The structure was solved from the model of Robertson & Woodward (1938). The coordinates of seven C atoms at y ~ 0 were used to synthesize a Fourier map. This revealed the positions of 13 of the 14 C atoms. Successive difference syntheses improved the coordinates of all the C atoms and gave R = Z [ i F o l IFcli/E IFot = 0.308. At this point a full-matrix leastsquares refinement with the program ORFLS (Busing, Martin & Levy, 1962) and unit weights lowered R to 0.141. A difference map revealed the positions of all H atoms. They were assigned isotropic temperature factors 1.25 /~2 greater than those of the C atoms to which they were bonded. The resultant structure factor calculation had an R of 0.128. A least-squares cycle on the C atom coordinates and the scale factor lowered R to 0-I12. Anisotropic thermal parameters were introduced, and two cycles of refinement, one on anisotropic thermal parameters and one on the positional parameters of the C atoms, reduced R to 0.085. The coordinates of the H atoms were improved by another difference synthesis and R dropped to 0.072. The refinement was continued by weighted [w = 1/a(F)] least squares until the parameter shifts were insignificant compared with the estimated standard deviations.

On the dipole moment of the ground state X 3Δ of iron carbide, FeC

In the light of experimental results on the dipole moment of the FeC X 3Δ state, we have re-examined our recent theoretical numbers of this property, by increasing our basis set size and calculating also the dipole moment by the finite field method. Our best result is 1.94 D as compared to the experimental value of 2.36 D, signifying that care should be exercised in obtaining one-electron properties even from highly correlated wave functions.

First principles study of the electronic structure and bonding of Mn2

We have examined the electronic structure and bonding of the Mn(2) molecule through multireference variational calculations coupled with augmented quadruple correlation consistent basis sets. The Mn atom has a (6)S(4s(2)3d(5)) ground state with its first excited state, (6)D(4s(1)3d(6)), located 2.145 eV higher. For all six molecular states (1)Sigma(g)(+), (3)Sigma(u)(+), (5)Sigma(g)(+), (7)Sigma(u)(+), (9)Sigma(g)(+), and (11)Sigma(u)(+)(1) correlating to Mn((6)S)+Mn((6)S), and for six undecets, i.e., (11)Pi(u), (11)Sigma(g)(+), (11)Delta(g), (11)Delta(u), (11)Sigma(u)(+)(2), and (11)Pi(g) with end fragments Mn((6)S)+Mn((6)D), complete potential energy curves have been constructed for the first time. We prove that the bonding in Mn(2) dimer is of van der Waals type. The interaction of two Mn (6)S atoms is hardly influenced by the total spin, as a result the six Sigma states, singlet ((1)Sigma(g)(+)) to undecet ((11)Sigma(u)(+)(1)), are in essence degenerate packed within an energy interval of about 70 cm(-1). Their ordering follows the spin multiplicity, the ground state being a singlet, X (1)Sigma(g)(+), with binding energy D(e) (D(0)) approximately 600 (550)cm(-1) at r(e) approximately 3.60 A. The six undecet states related to the Mn((6)S)+Mn((6)D) manifold, are chemically bound with binding energies ranging from 3 ((11)Pi(g)) to 25 ((11)Pi(u))kcal/mol and bond distances about 1 A shorter than the states of the lower manifold, Mn((6)S)+Mn((6)S). The lowest of the undecets is of Pi(u) symmetry located 30 kcal/mol above the X (1)Sigma(g)(+) state.

On the ground states of CaC and ZnC: A multireference Brillouin–Wigner coupled cluster study

We test the recently developed state-specific multireference Brillouin–Wigner coupled cluster (MRBWCCSD) method against the single reference CCSD method by examining theoretically the competing X 3Σ− and 5Σ− states of the (experimentally unknown) isovalent calcium and zinc carbide diatomics (CaC, ZnC). At the CCSD level, CaC is “incorrectly” predicted to have a ground 5Σ− state; however, the MRBWCCSD treatment restores the correct state ordering, and improves significantly the energetics for both molecules. Further comparison with various single- and multireference treatments shows that the latter are absolutely necessary for obtaining meaningful results for the ground states in both molecules.

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.

First principles study of the ground and excited states of FeO, FeO+, and FeO−

Through a variety of highly correlated methods combined with large basis sets we have studied the electronic structure of FeO, FeO(+), and FeO(-). In particular, we have constructed complete potential energy curves for 48, 24, and 4 states for the FeO, FeO(+), and FeO(-) species, respectively, at the multireference level of theory. For all states examined we report energetics, common spectroscopic parameters, and dipole moments. Overall our results are in good agreement with experiment, but we have encountered as well interesting differences between experiment and theory deserving further investigation.

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.