RESUMEN
A new procedure for full conformational analyses comprising the statistical analysis of molecular dynamics trajectories was developed and applied. This method included a coordinate space for sampling using molecular dynamics simulations, reduction of dimensionality using tensor decomposition tools, determination of probability distributions in a reduced space, and finally the search for all of the strict extrema points of probability distributions. These extracted extrema points formed an initial guess for geometry optimization and clustering of conformers. A complete conformational space of 1-oxaspiro[2,5]octane and its cis- and trans-4-, 5- and 6-methyl substituted derivatives was also determined. In each case, eight conformers were found with two chair-like conformers predominant at room temperature. It was found that chair-like conformers with an epoxide ring oxygen atom in the pseudo-axial position had less strain, as well as all of their conformers with the methyl substituent in an equatorial position on a cyclohexane moiety.
RESUMEN
Using non-equilibrium molecular dynamics simulations, it has been recently demonstrated that water molecules align in response to an imposed temperature gradient, resulting in an effective electric field. Here, we investigate how thermally induced fields depend on the underlying treatment of long-ranged interactions. For the short-ranged Wolf method and Ewald summation, we find the peak strength of the field to range between 2 × 10(7) and 5 × 10(7) V/m for a temperature gradient of 5.2 K/Å. Our value for the Wolf method is therefore an order of magnitude lower than the literature value [J. A. Armstrong and F. Bresme, J. Chem. Phys. 139, 014504 (2013); J. Armstrong et al., J. Chem. Phys. 143, 036101 (2015)]. We show that this discrepancy can be traced back to the use of an incorrect kernel in the calculation of the electrostatic field. More seriously, we find that the Wolf method fails to predict correct molecular orientations, resulting in dipole densities with opposite sign to those computed using Ewald summation. By considering two different multipole expansions, we show that, for inhomogeneous polarisations, the quadrupole contribution can be significant and even outweigh the dipole contribution to the field. Finally, we propose a more accurate way of calculating the electrostatic potential and the field. In particular, we show that averaging the microscopic field analytically to obtain the macroscopic Maxwell field reduces the error bars by up to an order of magnitude. As a consequence, the simulation times required to reach a given statistical accuracy decrease by up to two orders of magnitude.
RESUMEN
Analysis of spectral distribution of four commercial light sources for polymerisation of light cured composite materials is presented. Particular attention is paid to the valve lengths about 400 nm, as a critical area responsible for the possible eye damages.
