Per E. M. Siegbahn

Participated in the early development of the direct CI method, in particular with one UHF reference determinant. Contributed to the early development of UMP2 and UMP3 methods and suggested size consistency corrections. The first general direct multi-reference CI method based on the use of the unitary group approach. The first direct CI method for full CI with spin adapted configurations. Participated in the development of the Complete Active Space SCF (CASSCF) method. Suggested and developed the externally contracted CI method first for single then for multireference cases. Contributed to the internally contracted CI method.

A major activity area has been the development of computational strategies for accurate calculations. Obtained (with Bowen Liu) the first quantitatively correct three-dimensional potential energy surface for H3. Contributed during the last 15 years to the area of transition metal chemistry. Explained the oscillatory behaviour of chemisorption energies of adsorbates on transition metal cluster models of surfaces and introduced the concept of bond-preparation in this area. Found a new type of electronic shell closing effect for metal clusters with adsorbates. Suggested and developed the PCI-X scheme (parameterized CI with parameter X) particularly designed for accurate applications in transition metal chemistry.