Hinne Hettema

The Dalton quantum chemistry program system

Dalton is a powerful general‐purpose program system for the study of molecular electronic structure at the Hartree–Fock, Kohn–Sham, multiconfigurational self‐consistent‐field, Møller–Plesset, configuration‐interaction, and coupled‐cluster levels of theory. Apart from the total energy, a wide variety of molecular properties may be calculated using these electronic‐structure models. Molecular gradients and Hessians are available for geometry optimizations, molecular dynamics, and vibrational studies, whereas magnetic resonance and optical activity can be studied in a gauge‐origin‐invariant manner. Frequency‐dependent molecular properties can be calculated using linear, quadratic, and cubic response theory. A large number of singlet and triplet perturbation operators are available for the study of one‐, two‐, and three‐photon processes. Environmental effects may be included using various dielectric‐medium and quantum‐mechanics/molecular‐mechanics models. Large molecules may be studied using linear‐scaling and massively parallel algorithms. Dalton is distributed at no cost from http://www.daltonprogram.org for a number of UNIX platforms.

Frequency‐dependent polarizabilities and first hyperpolarizabilities of H2O

Static and frequency‐dependent dipole polarizabilities α and first hyperpolarizabilities β are calculated for H2O using self‐consistent field (SCF) and multiconfigurational self‐consistent‐ field (MCSCF) linear and quadratic response theory. With an active orbital space where one correlating orbital is included for each occupied valence orbital excellent agreement is obtained with the experimental hyperpolarizability. Basis set dependency has been investigated and a detailed vibrational analysis has been carried out.

The direct Monte Carlo method applied to the homogeneous nucleation problem

We discuss the application of the direct Monte Carlo method to the theory of cluster formation. Fractal relationships for the kernels appearing in the Smoluchowski equation are implemented in this method and the scaling behavior of the kernels is investigated using computer simulation. We study the effects of cluster disintegrations and also investigate the effects of ‘‘magic’’ numbers in cluster formation.

Models for Quantum Chemistry

This chapter provides the basic concepts for the characterisation of quantum chemistry as a research programme. I start with the characterisation of a quantum mechanics, and then gradually add the components that turn a generic quantum mechanics into quantum chemistry. This approach allows two specific philosophical outcomes: it allows us to make explicit what type of further characterisations are necessary to turn quantum mechanics into quantum chemistry and it allows us to specify in sufficient detail what the structure of quantum chemistry qua explanatory theory is.

Quantum Chemistry as a Research Programme

This chapter characterises quantum chemistry, the main reducing agent in chemistry, as a Lakatosian research programme. The attraction of the concept of a research programme is that it allows us to think of quantum chemistry not as a single theory, but as a succession (or network) of co-operating theories. This in turn allows for a more balanced discussion on how quantum chemistry can be characterised as a reducing theory. It also allows us to consider a number of questions on the internal structure of quantum chemistry, such as the question whether quantum chemistry is a progressive or degenerating research programme.

Unity of Chemistry and Physics: The Theory of Absolute Reaction Rates

The reduction of Eyring’s theory of absolute reaction rates is an interesting case study in chemistry. The theory reduction does not rely on a single ‘reducing’ theory, but on a network of multiple connected theories, each providing a piece of the answer. This situation is fairly common in chemistry, and this chapter provides an in-depth discussion of how we might think about this state of affairs as a ‘reduction’.

Molecular Structure: What Philosophers Got Wrong

The problems with the Born-Oppenheimer approximation have long been seen by the philosophy of chemistry community as indicative for the irreducibility of chemistry to physics. In this chapter I focus on this confusing discussion. The ‘derivation’ of molecular structure provided by the Born-Oppenheimer approximation is not ‘smooth’ and critically relies on a number of steps that are questionable. Yet to conclude from this that molecular structure is irreducible, as some philosophers have done, is going too far. This problem is largely unsettled in both its technical detail as well as its philosophical justification, and in this chapter I provide an overview of which questions are relevant to progress further research in this area.

Explaining the Chemical Bond: Idealisation and Concretisation

In this chapter, I focus on the ‘chemical bond’ – the phenomenon of bonding between two atoms – as an example of potential Nagelian reduction. There are two main competing theories, the Molecular Orbital and the Valence Bond theory, which provide a view from quantum mechanics on the phenomenon of bonding. The efficacy of both theories can be compared. It is furthermore interesting that there are systemic ways in which these theories can be improved, leading to the same computational view on the phenomenon of bonding. This means that the resulting structure is an idealisation/concretisation pair.

Reduction: Its Prospects and Limits

In this chapter, I discuss the concept of Nagelian theory reduction. In particular, I argue that the concept of Nagelian theory reduction as consisting of identities and (strict) derivation is naive, and not in keeping with both Nagel’s intention and writing on the topic. Instead, this concept of reduction seems to be given by an overly strict ‘metaphysical’ view on reduction in which reduction is only meaningful if and when the reduced theory is fully subsumed under the reducing theory.

Networks of Structures in Chemistry

In this chapter the focus is on the practical implications of the formal reconstructions in the previous two chapters. I specifically focus on the formal structures involved in reductions three examples: the periodic table of the elements, bonding, and the absolute theory of reaction rates. In all these three cases it will prove to be the case that formal connections exists between the reduced and reducing theories in the form of inter-theoretic links. Practically, the form of these links is situational and adapted to the specific case at hand. This implementation of reduction shows us that reduction is primarily an affair of practical science, and one that can only be said to have limited consequences in terms of an overarching ‘grand’ architecture of science: science is best conceived as a complex network of theories, where the links between these theories do enough work to be reductive in the liberal Nagelian sense discussed in Chap. 1, but no more.