Models in quantum chemistry

HERNÁN ACCORINTI; JUAN CAMILO MARTÍNEZ GONZÁLEZ

Leuven

Workshop; Leuven-Buenos Aires workshop on Philosophy of Physics; 2016

University of Leuven.

p { margin-bottom: 0.25cm; direction: ltr; line-height: 120%; text-align: left; }p.western { font-size: 12pt; }p.cjk { font-size: 12pt; }p.ctl { font-size: 12pt; }The purpose ofthe present paper is to participate in the debate in two interrelatedsteps:(i) First, weanalyze the discussions around the London brothers? model from acritical viewpoint. Our aim is to show that the debate has reached akind of dead end as the consequence of disagreements about theinterpretation of the very notion of independence and its role in theconstitution of scientific models.(ii) Second,we intend to contribute to find a way out of the dead end byappealing to a new example, not yet sufficiently discussed in thecurrent literature: the case of the molecular models used in quantumchemistry. Those models integrate two incompatible theoreticaldomains: quantum and classical. Quantum theory provides theSchrödinger equation to determine the energy levels of the molecule.The classical domain, through structural chemistry, establishes thegeometry of the molecule, given by the fixed position of the nucleiin space. The ?clamped-nucleus approximation?, introduced byBorn-Oppenheimer (1927), is incompatible with the Heisenberguncertainty principle, which establishes the impossibility to assignsimultaneously a definite position and a definite momentum to aquantum particle (see Hughes 1989).The analysisof the molecular models of quantum chemistry supplies a newperspective to address the problem of the relation between theoriesand models. This perspective shows that the independence of modelsfrom theories cannot be considered, as the traditional view holds, asa merely relative and historical situation that will be overcome withfurther theoretical development. By contrast, the case of models inquantum chemistry reveals a conceptual independence that isconstitutive of the modeling process. The existence of models thatintegrate incompatible theories constructively and in an empiricallysuccessful manner provides an argument to call into question thetraditional view, according to which a model is always a model of acertain theory, on which it depends.