and In the IAU Symposium of 1979 devoted to interstellar molecules [8]. Excellent relevant monographs [ 9. 10] . related timely proceedings [ 11] . and recently published elementary textbooks [12. 13] further help to define the pedagogical scope of molecular astrophysics. A significant financial investment has been made in the establishment of ground- and satellite-based observationai facilities for molecuiar astrophysical studies. In the coming years. a wealth of experimental data is bound to accumulate. in which connection close interactions between observers. astrophysical modeliers. and molecular physicists and chemists can play a helpful role in analysis and interpretation. In view of the increasing pace of activity in the field of molecular astrophysics. and in the apparent absence of relevant international meetings since the Liege 1977 and IAU 1979 Symposia. it was deemed appropriate and timely by the organizers to hold a workshop in 1984. Consequently. the NATO Advanced Research Workshop. "Molecular Astrophysics State of the Art and Future Directions". was organized and held at Bad Wlndshelm. West Germany. from 8 to 14 July 1984. The choice of speakers and subject matter of the Workshop was largely subjective. but designed to include most of the generally accepted areas of molecular astrophysical study. Workers from the fields of radio. infrared. and uv-optlcal observations. astrophysical modelling. laboratory spectroscopy. reaction chemistry. collision physics. and theoretical molecular physics and chemistry. were Invited to present survey lectures In their areas of speciality. In addition.

This volume records the lectures given at a NATO Advanced Study Institute on Methods in Computational Molecular Physics held in Bad Windsheim, Germany, from 22nd July until 2nd. August, 1991. This NATO Advanced Study Institute sought to bridge the quite considerable gap which exist between the presentation of molecular electronic structure theory found in contemporary monographs such as, for example, McWeeny's Methods 0/ Molecular Quantum Mechanics (Academic Press, London, 1989) or Wilson's Electron correlation in moleeules (Clarendon Press, Oxford, 1984) and the realization of the sophisticated computational algorithms required for their practical application. It sought to underline the relation between the electronic structure problem and the study of nuc1ear motion. Software for performing molecular electronic structure calculations is now being applied in an increasingly wide range of fields in both the academic and the commercial sectors. Numerous applications are reported in areas as diverse as catalysis and interstellar chernistry, drug design and environmental studies, molecular biology and solid state physics. The range of applications continues to increase as scientists recognize the importance of molecular structure studies to their research activities. Recent years have seen a growing dependence of these applications on program packages, which are often not in the public domain and which may have a somewhat lirnited range of applicability dicta ted by the particular interests and prejudices of the program author.

This NATO Advanced Study Institute was concerned with modern ab initio methods for the determination of the electronic structure of molecules. Recent years have seen considerable progress in computer technology and computer science and these developments have had a very significant influence on computational molecular physics. Progress in computer technology has led to increasingly larger and faster systems as well as powerful minicomputers. Simultaneous research in computer science has explored new methods for the optimal use of these resources. To a large extent develop ments in computer technology, computer science and computational molecular physics have been mutually dependent. The availability of new computational resources, particularly minicomputers and, more recently, vector processors, has stimulat'ed a great deal of research in molecular physics. Well established techniques have been reformulated to make more efficient use of the new computer technology and algorithms which were previously computationally intractable have now been successfully implemented. This research has given a new and exciting insight into molecular structure and molecular processes by enabling smaller systems to be studied in greater detail and larger systems to be studied for the first time."

This book contains the transcripts of the lectures presented at the NATO Advanced study Institute on "Computational Techniques in Quantum Chemistry and Molecular Physics," held at Ramsau, Germany, 4th - 21st Sept. 1974. Quantum theory was developed in the early decades of this century and was first applied to problems in chemistry and molecular physics as early as 1927. It soon emerged however, that it was impossible to con sider any but the simplest systems in any quantita tive detail because of the complexity of Schrodinger's equation which is the basic equation for chemical and molecular physics applications. This remained the si tuation until the development, after 1950, of elec tronic digital computers. It then became possible to attempt approximate solutions of Schrodinger's equa tion for fairly complicated systems, to yield results which were sufficiently accurate to make comparison with experiment meaningful. Starting in the early nineteen sixties in the United States at a few centres with access to good computers an enormous amount of work went into the development and implementation of schemes for approximate solu tions of Schrodinger's equation, particularly the de velopment of the Hartree-Fock self-consistent-field scheme. But it was soon found that the integrals needed for application of the methods to molecular problems are far from trivial to evaluate and cannot be easily approximated."

For all practical purposes the basic physical equations governing the behaviour of a system at the molecular level can only be solved approximately. The key issue in any reliable and accurate computational study in molecular physics and quantum chemistry is the adoption of a suitable model which contains the essential physics and chemistry, is computationally tractable, and preferably amenable to systematic refinement. The provision of advice on the choice of an appropriate model for a specific problem has so far received scant attention. This issue is becoming acute as standard' software packages are becoming widely available and are being increasingly heavily used in both the academic and industrial sectors by researchers who have received no special training in the theoretical physics and chemistry that underpins them. This volume provides researchers whose background may not be in the computational molecular sciences with the necessary background to make intelligent use of the methods available by performing reliable calculations of appropriate accuracy and making a considered interpretation of the data so obtained.

This NATO Advanced Study Institute was concerned with modern ab initio methods for the determination of the electronic structure of molecules. Recent years have seen considerable progress in computer technology and computer science and these developments have had a very significant influence on computational molecular physics. Progress in computer technology has led to increasingly larger and faster systems as well as powerful minicomputers. Simultaneous research in computer science has explored new methods for the optimal use of these resources. To a large extent develop ments in computer technology, computer science and computational molecular physics have been mutually dependent. The availability of new computational resources, particularly minicomputers and, more recently, vector processors, has stimulat'ed a great deal of research in molecular physics. Well established techniques have been reformulated to make more efficient use of the new computer technology and algorithms which were previously computationally intractable have now been successfully implemented. This research has given a new and exciting insight into molecular structure and molecular processes by enabling smaller systems to be studied in greater detail and larger systems to be studied for the first time."

This book contains the transcripts of the lectures presented at the NATO Advanced study Institute on "Computational Techniques in Quantum Chemistry and Molecular Physics," held at Ramsau, Germany, 4th - 21st Sept. 1974. Quantum theory was developed in the early decades of this century and was first applied to problems in chemistry and molecular physics as early as 1927. It soon emerged however, that it was impossible to con sider any but the simplest systems in any quantita tive detail because of the complexity of Schrodinger's equation which is the basic equation for chemical and molecular physics applications. This remained the si tuation until the development, after 1950, of elec tronic digital computers. It then became possible to attempt approximate solutions of Schrodinger's equa tion for fairly complicated systems, to yield results which were sufficiently accurate to make comparison with experiment meaningful. Starting in the early nineteen sixties in the United States at a few centres with access to good computers an enormous amount of work went into the development and implementation of schemes for approximate solu tions of Schrodinger's equation, particularly the de velopment of the Hartree-Fock self-consistent-field scheme. But it was soon found that the integrals needed for application of the methods to molecular problems are far from trivial to evaluate and cannot be easily approximated."