Sparkles in Mopac : summary

 Dear Netters,
  here is a summary of the messages I received after my original posting about
  the use of sparkles in Mopac.
  Original posting :
  Dear Netters,
  I would like to calculate with MOPAC the geometry of an organic anion.
  I am not sure that the calculation of an ion alone in the gas-phase
  would be very realistic (mainly because of intramolecular electrostatic
  repulsion), so I tried to use the so-called "sparkles" which are
 defined in
  the MOPAC manual as "pure ionic charges" or more precisely as an
  charge at the center of a repulsion sphere of form exp(-alpha*r)".
  The manual explains further that the sparkles can be used as counterions.
  Unfortunately, my MOPAC calculation with a positive sparkle "+" ended
  with the positive charge almost superimposed on the atom of my anion bearing
  most of the negative charge.
  Have I missed something in the keyword specification ? does anybody have
  experienced this MOPAC feature with some success ? (some refs would be
  welcome ).
  I managed to fix my problem when I realized that I was starting the geometry
  optmization with a too short distance between the sparkle and the anion. This
  caused the sparkle to nearly overlap the molecule in the end. Starting with a
  distance longer than the supposed radius of the sparkle allowed convergence to
  a geometry where the sparkle stays at a reasonable distance from the anion.
  Thanks to all who replied to me.
  Remi Le Goas
  Computer Aided Molecular Design
  Rhone-Poulenc Agrochimie
  LYON - France
  e-mail: remi.le-goas #*at*#
  from jstewart #*at*# :
  Please check that your copy of MOPAC has the following bug-fixes:
                                                                      17 June
  ++++  In MOLDAT, after
           CALL CALPAR
        DO 91 I=1,107
    91  IF(AM(I).LT.1.D-4)AM(I)=1.D0
  Reason:  Sparkles were not being handled correctly
                                                                   8 January 1994
  ++++ In block.f, after
        DATA VS(104)       /10.0D0/
        DATA ALPAM1(104)   / 1.5D0/
  Reason: The AM1 sparkle `+'  was not correctly specified.
  James J. P. Stewart
  Dear Dr. Le Goas,
    Sparkles are not really quantum mechanical objects, but are simply
  acting as "polarizers".  They have no electrons and is the geometry
  optimized with one being used, it will simply move toward the negative
  charge as you noted.  If you want to try computing the geometry of
  an organic anion as if it were in solution, I would use one of the semi-
  empirical continuum models.  In modern versions of MOPAC I think that
  this is COSMO.  In our AMPAC proogram, it is AMSOL.  I will be happy to
  send you info on that program if you would like.  This is not perfect,
  but should be better that a "bare" anion.
                            DR. ANDREW HOLDER
           Assistant Professor of Computational/Organic Chemistry
  Department of Chemistry          **  Internet Addr: aholder #*at*#
  Univ. of Missouri - Kansas City  **  Phone Number:  (816) 235-2293
  Spencer Chemistry, Room 315      **  FAX Number:    (816) 235-5502
  Kansas City, Missouri 64110      **
  Dear Dr Goas,
  It seems that the program worked correctly. Most probably you have
  asked the program to optimize the distance between the sparkle and the
  anion (or the atom concerned). Try to fix the distance (to see the effect
  of the potential created by the sparkle (or sparkles). But, as it is
  written in the Mopac Manual, you have to be very careful when using
  them. You could alternatively use COSMO within MOPAC if you do not
  wish to handle isolated anion species.
      I had problems getting rezonable results using sparkles when
  I tried to mimic iron atom within ferrocenophanes (J.Phys. Org. Chem.
  1989,2,602-610) and then decided to develop my own molecular
  mechanics force field for this species (J.Phys.Org. Chem. 1992,5,382-394).
      You could also ask for help to MOPAC Forum that was opened
  for a while at no cost. Then you should get help from Dr Stewart directly.
  Wishes, Jerzy Rudzinski
  Dr Jerzy M Rudzinski
  Fujitsu Kyushu System
  Fukuoka, Japan            jerzy #*at*#
  Here is a sparkles job that i've run.  It is not an acid counter ion
  but coordinates to the carbonyl of acrylic acid.  The results of the
  four isomers of acrylic acid coordinated with the potassium sparkle
  mimic the corresponding results of ab initio calculations
  coordinated with Li+.  This case should give you an idea of the use
  of the sparkle and can help you to determine if there are problems
  with your code.
  rjl #*at*#    internet address
  AM1 GNORM=0.001 SCFCRT=1.D-16 T=9999M HESS=1 EF CHARGE=1
   acrylic acid cis,syn conformation coordinated with K+
  FINAL HEAT OF FORMATION =        -62.04709 KCAL
    C    0.000000  0    0.000000  0    0.000000  0    0    0    0      -0.0565
    C    1.329716  1    0.000000  0    0.000000  0    1    0    0      -0.2014
    H    1.087972  1  123.151621  1    0.000000  0    1    2    0       0.0986
    H    1.086237  1  122.310316  1  180.000000  0    1    2    3       0.0746
    C    1.482676  1  121.909848  1    0.000000  0    2    1    3       0.2569
    H    1.097405  1  120.693802  1  180.000000  0    2    1    3       0.1048
    O    1.228531  1  125.734878  1    0.000000  0    5    2    1      -0.3987
    O    1.396593  1  117.806960  1  180.000000  0    5    2    1      -0.0089
    H    0.989783  1  120.953680  1  180.000000  0    8    5    2       0.0557
    K    2.989783  1  120.953680  1  180.000000  0    7    5    8       0.0557
    0    0.000000  0    0.000000  0    0.000000  0    0    0    0
     Dear Dr. Remi,
   Please take a look at our paper entitled:
         "Sparkle Model for the Quantum Chemical Calculation of Lanthanide
          Chem. Phys. Lett. 227 (1994) 349-353.
   There you will find a description of how to optimize the sparkle
  parameters. It may be of help to you. I guess that you may have to
  reparametrize them to suit your needs.
  *                                                                      *
  * Alfredo Mayall Simas                     e-mail: AMS #*at*#    *
  * Departamento de Quimica Fundamental         Fax: (081) 271-8442      *
  * Universidade Federal de Pernambuco       Office: (081) 271-8440 x 37 *
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