MOLECULAR ELECTRON-DENSITY LEGO APPROACH TO MOLECULE BUILDING

A new method is presented for the construction of ab initio quality ap proximate electronic charge distributions for large molecules from cha rge distributions of small molecular fragments. This method is reminis cent to building structures using Lego blocks. The electronic density distribution calculated using the method is quantitatively shown to be very similar to that calculated for entire molecules using convention al ab initio packages with standard basis sets such as 6-31G*, while requiring only a fraction of the computational time. The pre-calculate d fuzzy electron distributions of base molecular fragments, stored in a data bank, are merged (rotated, translated, and subsequently added t ogether) to calculate these approximate charge distributions for the m olecule. The method requires only the Cartesian coordinates of the nuc lei for the construction of molecular charge distributions. The speed of the method makes it potentially useful for molecular modeling appli cations where construction of electron densities of large molecules is especially important. The technique can be used to replace fused sphe re models of large molecules with more realistic electron density cont ours. One additional advantage is that these contours can be displayed for a whole range of density values, from tight, high-density contour s, representing the bonding pattern, to loose, low-density contours re presenting the peripheral shape and size features of molecules.