Potential energy surfaces governing chemical reactions involving carbon, oxygen and hydrogen

The lowest singlet states of O[subscript]3 in C[subscript] 2v are studied in the Full Optimized Reaction Space (FORS) MCSCF level of theory with an extended atomic basis set plus polarization functions. The [superscript]1A\u27 ground state potential energy surface contains two minima. The upper minimum lies 29.8 kcal/mole above the ground state minimum and most importantly above the O[subscript]2([superscript]3[sigma][subscript]g[superscript]-) + O([superscript]3P) dissociation limit. It resembles a ring structure having D[subscript] 3h symmetry. The potential energy surface governing the C[subscript] 2v restricted ring opening of the cyclic O[subscript]3 to the ground state is also computed. A conical intersection is found between the 1-[superscript]1A[subscript]1 and 2-[superscript]1A[subscript]1 potential energy surfaces. This first case of an intersection of two states of the same symmetry in a real system is definitively proved by monitoring the sign of the wavefunction on a closed loop around it;Ab-initio calculations elucidating the structure, the ring opening and the dissociation process of the cyclic CO[subscript]2 isomer are reported. The optimal isosceles-triangle (C[subscript] 2v) geometries corresponding to the C[subscript] 2v constraint dissociation OCO → C + O[subscript]2 are determined. The entire C[subscript] 2v surface is computed, revealing the existence of a metastable cyclic carbene-type species corresponding to a local minimum 137.6 kcal/mole above the linear total minimum. Finally, energies are determined for various relevant cross sections with lower symmetry (C[subscript] s), i.e. for asymmetric bond lengths;Extended basis set calculations for the key regions of the ground state [superscript]1A[subscript]1 cyclopropylidene (C[subscript] 2v) to allene (D[subscript] 2d) ring opening reaction surface are performed within the FORS MCSCF framework. Optimized geometries of the reactant, product, transition state and allene isomerization transition state as well as the barrier for the ring opening and the allene isomerization together with the overall exothermicity are reported in the various levels of MCSCF approximation incorporating FORS spaces ranging from 20 to 1764 configurations. The reaction path from the transition state passes from a point where the two surfaces corresponding to the [superscript]1A\u27 and [superscript]1A\u27\u27 states intersect each other. Explanations for the various features of the potential energy surface governing the ring opening of cyclopropylidene to allene are obtained through localized quasi-atomic FORS MO\u27s. ftn*Performed under Contract No. W-7405-Eng-82 for the U.S. Dept. of Energ

The Melting Temperature of Liquid Water with the Effective Fragment Potential

The direct simulation of the solid–liquid water interface with the effective fragment potential (EFP) via the constant enthalpy and pressure (NPH) ensemble was used to estimate the melting temperature (Tm) of ice-Ih. Initial configurations and velocities, taken from equilibrated constant pressure and temperature (NPT) simulations at P = 1 atm and T = 305 K, 325 K and 399 K, respectively, yielded corresponding Tm values of 378 ± 16 K, 382 ± 14 K and 384 ± 15 K. These estimates are consistently higher than experiment, albeit to the same degree as previously reported estimates using density functional theory (DFT)-based Born–Oppenheimer simulations with the Becke-Lee–Yang–Parr functional plus dispersion corrections (BLYP-D)

Computational Thermochemistry and Benchmarking of Reliable Methods

During the first and second years of the Computational Thermochemistry and Benchmarking of Reliable Methods project, we completed several studies using the parallel computing capabilities of the NWChem software and Molecular Science Computing Facility (MSCF), including large-scale density functional theory (DFT), second-order Moeller-Plesset (MP2) perturbation theory, and CCSD(T) calculations. During the third year, we continued to pursue the computational thermodynamic and benchmarking studies outlined in our proposal. With the issues affecting the robustness of the coupled cluster part of NWChem resolved, we pursued studies of the heats-of-formation of compounds containing 5 to 7 first- and/or second-row elements and approximately 10 to 14 hydrogens. The size of these systems, when combined with the large basis sets (cc-pVQZ and aug-cc-pVQZ) that are necessary for extrapolating to the complete basis set limit, creates a formidable computational challenge, for which NWChem on NWMPP1 is well suited