Chronic Intestinal Stasis.

S 1 –S vibronic spectra of benzene clusters revisited. I. The tetramer

“…Theoretical studies on the trimer have predicted that the most stable structure has cyclic geometry where three benzenes are arranged so as to form an equilateral triangle. 29,39,48,57,59,63 This structure is consistent with IDSRS results reported by Felker's group. 50,53 Here we shall briefly describe their experimental method since this information is necessary to discuss our results.…”

“…The structure with S 4 symmetry has been confirmed as the global minimum by Williams 48 and Pullan, 57 who applied the genetic algorithm to the minimum-search procedures on the same model potential.…”

“…53 In addition, in the case of the S 4 symmetric tetramer, a librational mode belonging to A symmetry gains no intensity, because the total cluster anisotropy is not modulated by it ͑as shown for the naphthalene trimer with C 3h symmetry͒. 83 The above consideration combined with the theoretical results 29,48,57,59 provides us with a coherent description of the gross feature of the intermolecular Raman spectra. First, the less congestion in the tetramer spectrum should owe primarily to fewer vdW modes ͑14 in total͒ in the S 4 tetramer because of fewer intermolecular degrees of freedom ͑relative to the larger clusters͒ in addition to the degeneracy in E modes.…”

Ab initio molecular packing analysis

“…It is extremely unlikely that the hessian of a random trial structure will be positive definite. More likely, a random point is near one of the many saddle points of the energy surface and the fastest way to go to lower energy is to move in the direction of eigenvectors corresponding to negative eigenvalues; this is the OREM method previously found useful in obtaining global energy minima of benzene molecular clusters (Williams, 1992). OREM shifts usually yield a large energy decrement; if this is not the case, mpa can automatically switch to steepest-descents (SD) minimization.…”

“…For example, in early benzene molecular-cluster calculations, Williams (1980) assumed inversion symmetry and limited consideration to clusters containing an odd number of molecules. More recently, symmetry constraints were removed and four different energyminimum structures for the benzene tetramer cluster have been predicted (Williams, 1992). In the earlier work, 13-and 15-molecule clusters showed some resemblance to the structure of crystalline orthorhombic benzene.…”