Adamantane

Adamantan has been generated and optimized with Kimball.exe (G2Fix.prf prefactors, containing some correlation energy) using 10 variables: Five Kimball radii (C1(1s), C2(1s), C1-C2, C1-H, C2-H), four angles (C2C1C2, C1C2C1, HC1H, HC2H), and the proton excentricity. Axial and equatorial C-H bonds are assumed to be equal (could be changed). The program finds, for the given tolerance, an energy minimum with C1 symmetry which is within 0.001Å and 0.2 deg of Td symmetry. The total energy is slightly better than that of an SCF-calculation in the HF/4-21g basis -387.22360, and similar to a HF/6-31g, -387.85749 or a HF/6-31+g(2d,p) computation: -388.06419. The latter three are based on optimized structures with Gaussian98 starting from the given Kimball structure (relaxed to smaller than 0.0005 mdyn for all displacement forces). With polarization functions the correct symmetry group Td is reached (symm=loose). For Kimball.exe the time from the end of the graphical setup of the molecule to reach the final result is 30 sec on a Pentium 233 MHz. The stereoplot shows the sequence of the atoms (numbers on symbols) during the construction of the molecule with the node shown in the inset, lower right corner: C1→C2, C1→C3, C2→C4, C3→C5, C4→C6, ring closure C6→C5; C1→C7→C8→C9, ring closure C9→C4; C8→C10, ring closure C10→C5. The H-atoms are added automatically. Mopac7 finds a formation enthalpy at 298K of -38.9 kcal/mol with the AM1 hamiltonian and "Gradients were initially acceptably small, SCF field reached" using the Kimball structure.