From mxm@iris80.biosym.com Fri Sep 10 14:02:50 1993 Date: Fri, 10 Sep 93 21:02:50 -0700 From: mxm@biosym.com (Max Muir ) Message-Id: <9309110402.AA22850@iris80.biosym.com> To: chemistry@ccl.net Subject: MOPAC6.0 ESP, Again Dear CyberChemists, Calcn. of ESP fitted charges with MOPAC6.0 _________________________________________ a collegue has been using MOPAC6.0's ESP option to get a set atom charges fitted (least squares) to a bunch of electrostatic potential points on Connolly surfaces at 1.4, 1.6,1.8 and 2.0 times the vdWs atom radius. Old hat, yes, but he is unable to reproduce the values that appear in Kolman's paper (J. Comp. Chem., Vol. 11, No. 4, 431- 439 (1990)) and neither am I. Consider the case of formaldehyde (does anyone call it methanal?). I'm using the default values for SCALE (1.4) NSURF(4) and SCINCR (0.2). Here's my input deck . . GRAPH NODIIS MMOK ESP SLOPE=1.422 POTWRT NOINTER PRECISE I shot an elephant in my pyjamas the other day C 0.000000 0 0.000000 0 0.000000 0 0 0 0 O 1.230000 1 0.000000 0 0.000000 0 1 0 0 H 1.090000 1 120.000092 1 0.000000 0 1 2 0 H 1.070000 1 120.000114 1 180.000000 1 1 2 3 The fitted charges are . . (from .out) ELECTROSTATIC POTENTIAL CHARGES ATOM NO. TYPE CHARGE SCALED CHARGE 1 C 0.3897 0.5541 2 O -0.3613 -0.5137 3 H -0.0142 -0.0202 4 H -0.0142 -0.0202 THE NUMBER OF POINTS IS: 375 THE RMS DEVIATION IS: 0.7655 THE RRMS DEVIATION IS: 0.0794 While Kollman et al report (in figure 1) the following MNDO charges. Formaldehyde H +0.006 C +0.339 O -0.352 The unscaled fitted charges aren't too bad, but the H's have changed sign. The scaled fitted charges seem incredible and your humble correspondent does not believe them. Have I missed something obvious? Yours faithfully, Max Muir