Trygve Helgaker
Born August 11, 1953 in Porsgrunn, Norway.
Professor of Chemistry, Department of Chemistry, University of Oslo, Norway.
Email:trygve.helgaker@kjemi.uio.no
Web: external link
A/S Norsk Varekrigsforsikring Fonds Prisbelønning, 1985; The Norwegian Academy of Science and Letters, 2004;
The International Academy of Quantum Molecular Science, 2005; Scientific Board of World Association of
Theoretical and Computational Chemists (WATOC), 2005
Author of:
More than 200 scientific papers, “Molecular ElectronicStructure Theory” (Wiley, Chichester, 2000),
with Poul Jørgensen and Jeppe Olsen. One of the principal developers of the Dalton program package.
Important Contributions:

The development of secondquantization theory for the calculation of response functions
with perturbationdependent basis functions; its implementation and application to geometrical and magnetic molecular properties.
The calculation of NMR parameters, including the calculation of indirect spinspin coupling constants
in molecules containing several hundred atoms by linearscaling densityfunctional techniques.
The unconstrained parameterization of the atomicorbital density matrix in selfconsistent field theories,
with applications to energy optimization and property evaluation.

The introduction of the variational Lagrangian method for the calculation of molecular properties
for nonvariational wave functions in the same manner as for variational wave functions (a generalization of the
Handy–Schaefer technique, applicable to dynamic as well as static perturbations),
with the wavefunction parameters obeying the 2n+1 rule and their multipliers the 2n+2 rule.

The development of the integraldirect coupledcluster method, making possible calculations
in very large basis sets. Its application to the study of the basisset convergence of orbitalbased correlated methods;
the establishment of the principal orbital expansion and the twopoint extrapolation technique,
typically reducing basisset errors by an order of magnitude, thereby making standard, orbitalbased calculations
competitive with explicitly correlated ones. The accurate and systematic benchmarking of quantum chemistry,
including the rigorous calculation of atomization energies and spectroscopic constants to within the errors
imposed by the Schrödinger equation.

The introduction of ab initio direct dynamics for integrating the classical Born–Oppenheimer
trajectories on the fly, without constructing the potential energy surface in advance, with several
applications using multiconfigurational selfconsistent field theory.