Applications of theoretical chemistry: Effective Hamiltonian studies of calcium hydroxide and implicit solvent simulations of poly(ethylene oxide)

This work encompasses two disparate areas related solely in that both are concerned with accurate determination of energetic contributions. The first part concerns ab initio electronic structure calculations employing the effective valence shell Hamiltonian ($Hv$) method on the CaOH radical. The second part focuses on implicit solvent molecular dynamics simulations for solvated synthetic macromolecules (polymers) at infinite dilution in aqueous environments.; $Hv$ calculations have been carried out on CaOH, an important transitional molecule in the alkaline-earth metal monohydroxide series, to track the excited (and ground) state surface response to the deformation of the molecular geometry. An avoided crossing as the molecule bends causes quasilinear and non-linear behavior in the excited states, especially the F˜ 2Π and G˜ 2Π states which are both found to have barriers to linearity at non-global minima. It is determined that basis sets for highly accurate calculations on the CaOH radical rely more upon the number of tight functions around the Ca atom than polarization or overall basis size. The CaOH radical demonstrates the power and viability of the $Hv$ method for the construction of global potential energy surfaces, especially for excited states.; The results of Langevin dynamics simulations for aqueous poly(ethylene oxide) chains (PEO₆, PEO₁₂, and PEO₅₄) are compared with all-atom (explicit water) simulations of the same length chains. A non-standard force field for PEO has been implemented in (S)TINKER, and an expanded set of atom classes developed for the solvation potential of ethers. Thus the absence of optimized force fields and potentials for the more “simple” synthetic polymers has been somewhat improved for PEO, and initial results show satisfactory agreement with the far more costly explicit solvent (all atom) simulations at this level. It is clear that the implicit solvent approach with a modified SASA potential has great promise as a tool for studying the long-time dynamics of polymers in solution.