Table Of ContentMethods in
Computational Chemistry
Volume 1
Electron Correlation
in Atoms and Molecules
METHODS IN COMPUTATIONAL CHEMISTRY
Volume 1 Electron Correlation in Atoms and Molecules
Edited by Stephen Wilson
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Methods in
Cotnputational Chetnistry
Volume 1
Electron Correlation
in Atoms and Molecules
Edited by
STEPHEN WILSON
University of Manchester Regional Computer Centre
Manchester, England
Springer Science+Business Media, LLC
Library of Congress Cataloging in Publication Data
Methods in computational chemistry.
Includes bibliographies and index.
Contents: v. 1. Electron correlation in atoms and molecules / edited by Stephen
Wilson.
1. Chemistry —Data processing. I. Wilson, S. (Stephen), 1950-
OD39.3.E46M47 1987 542 87-7249
ISBN 978-1-4899-1985-4
ISBN 978-1-4899-1985-4 ISBN 978-1-4899-1983-0 (eBook)
DOI 10.1007/978-1-4899-1983-0
© 1987 Springer Science+Business Media New York
Originally published by Plenum Press, New York in 1987
Softcover reprint of the hardcover 1st edition 1987
All rights reserved
No part of this book may be reproduced, stored in a retrieval system, or transmitted
in any form or by any means, electronic, mechanical, photocopying, microfilming,
recording, or otherwise, without written permission from the Publisher
Contributors
Ivan Cernusak, Department of Physical Chemistry, Comenius University,
Bratislava, Czechoslovakia
Karol Jankowski, Institute of Physics, Nicholas Copernicus University,
Toruil, Poland
Vladimir Kello, Department of Physical Chemistry, Comenius University,
Bratislava, Czechoslovakia
Jozef Noga, Institute of Inorganic Chemistry, Center for Chemical
Research, Slovak Academy of Sciences, Bratislava, Czechoslovakia
Miroslav Urban, Department of Physical Chemistry, Comenius Univer
sity, Bratislava, Czechoslovakia
B. H. Wells, Physical Chemistry Laboratory, University of Oxford,
Oxford, England
s.
Wilson, University of Manchester Regional Computer Centre,
Manchester, England
v
Foreword
When, forty years ago, as a student of Charles Coulson in Oxford
I began work in theoretical chemistry, I was provided with a Brunsviga
calculator-a small mechanical device with a handle for propulsion, metal
levers for setting the numbers, and a bell that rang to indicate overflow.
What has since come to be known as computational chemistry was just
beginning. There followed a long period in which the fundamental theory
of the "golden age" (1925-1935) was extended and refined and in which the
dreams of the early practitioners were gradually turned into hard
arithmetic reality. As a still-computing survivor from the early postwar
days now enjoying the benefits of unbelievably improved hardware, I am
glad to contribute a foreword to this series and to have the opportunity of
providing a little historical perspective.
After the Brunsviga came the electromechanical machines of the late
1940s and early 1950s, and a great reduction in the burden of calculating
molecular wavefunctions. We were now happy. At least for systems con
taining a few electrons it was possible to make fully ab initio calculations,
even though semiempirical models remained indispensable for most
molecules of everyday interest. The 1950 papers of Hall and of Roothaan
represented an important milestone along the road to larger-scale non
empirical calculations, extending the prewar work of Hartree and Fock
from many-electron atoms to many-electron molecules-and thus into
"real chemistry." But in a practical sense perhaps the most dramatic ~vent
was the appearance of the automatic digital computer-which ended an era
and opened a new one.
The first electronic computer I used was the "Whirlwind" at MIT in
1954; it solved my large (20 x 201) systems of secular equations in only a
few minutes, but high-level languages were not then available and "integral
packages" were nonexistent. Integrals over Slater orbitals were evaluated
using Barnett-Coulson expansion techniques (and desk machines) and a
single three-center two-electron integral might take a few hours of tedious
vii
viii Foreword
work. Merely computing the integrals in readiness for a "double-zeta"
calculation on the ammonia molecule (today within the reach of a personal
computer) would have taken about fifteen working years so we had to be
less ambitious.
Nowadays, ab initio calculations of near-Hartree-Fock accuracy, even
on quite large molecules, are regarded as "standard"; freely available
program packages have brought the application of sophisticated theories
within the reach of any chemist with access to (in today's terms) quite
modest computing power. And whereas thirty years ago the predictions of
theoretical chemists were greeted with, at best, skepticism or, at worst,
derision, there is now a danger that chemists with computers put too much
trust in their numerical results; for the magic words "ab initio" do not
confer some kind of guarantee that the output from a computer will be
significant, and programs are frequently used in situations quite outside
their range of applicability. Unless computing power is applied with
discrimination and understanding even the most elaborate calculations can
easily lead to nonsense.
There is a more fundamental problem in the interpretation of
numerical results and in the assessment of their true significance, one with
which the first volume in the series is concerned: the "correlation error,"
which remains even when a total electronic energy is computed at the
Hartree-Fock limit, may well be a thousand times greater than the
required barrier height or dissociation energy-so how can reliable predic
tions of anything be made? Many of the real innovations of theory in
recent years have to do with this so-called "correlation problem" but the
details are mathematically and conceptually demanding and are buried in a
vast literature. Techniques for removing the correlation error have not yet
been embodied in "standard" programs that can be used by the uninitiated.
Those which do exist make increasingly heavy demands on the user-he
will need a considerable appreciation of the limitations and characteristics
of the underlying theories; he will have to make intelligent choices of input
data and control parameters and to know something of the algorithms
used along the route from input to output; he will have to know what
questions it is reasonable to ask and how to interpret the "answers" which
emerge.
The appearance of this series-which aims at bringing advances in
theory, and especially their computational implementation, within the
reach of chemists who need to make calculations, whether on molecular
structure and behavior, on reaction dynamics, or on the bulk properties of
chemical systems-is timely and to be welcomed. As the subject of its first
volume, the choice of electron correlation in atoms and molecules-a topic
which has haunted quantum chemistry for many years and is now the sub
ject of massive computational effort-is particularly appropriate. This book
Foreword ix
is the work of experts, who write with authority and clarity on their chosen
fields, and forms a fitting start to the series. I wish it, and the volumes that
will follow, every success.
Roy McWeeny
Department of Chemistry
University of Pisa
Pisa, Italy
Preface
Today the digital computer is a major tool of research in chemistry and the
chemical sciences. However, although computers have been employed in
chemical research since their very inception, it is only in the past ten or
fifteen years that computational chemistry has emerged as a field of
research in its own right. The computer has become an increasingly
valuable source of chemical information, one which can complement and
sometimes replace more traditional laboratory experiments. The com
putational approach to chemical problems can not only provide a route to
information which is not available from laboratory experiments but can
also afford additional insight into the problem being studied, and, as it is
often more efficient than the alternatives, the computational approach can
be justified in terms of economics.
The applications of computers in chemistry are manifold. A broad
overview of both the methods of computational chemistry and their
applications in both the industrial research laboratory and the academic
research environment is given in my book Chemistry by Computer (Plenum
Press, 1986). Applications of the techniques of computational chemistry
transcend the traditional divisions of chemistry-physical, inorganic, and
organic-and include many neighboring areas in physics, biochemistry,
and biology. Numerous applications have been reported in fields as diverse
as solid-state physics and pesticide research, catalysis and pharmaceuticals,
nuclear physics and forestry, interstellar chemistry and molecular biology,
surface physics and molecular electronics. The range of applications con
tinues to increase as research workers in chemistry and allied fields identify
problems to which the methods of computational chemistry can be applied.
The techniques employed by the computational chemist depend on the
size of the system being investigated, the property or range of properties
which are of interest, and the accuracy to which these properties must be
measured. The methods of computational chemistry range from quantum
mechanical studies of the electronic structure of small molecules to the
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