Molecular electronic structure-theory calculations are mostly performed in the absence of magnetic fields, with the ability to include the effects of internal and external magnetic fields perturbatively, as corrections to the field-free description of the molecule. In the MOLMAG project, the molecules are instead studied in finite magnetic fields, in nonperturbative manner. Our purpose is twofold: to learn about molecules in magnetic fields and to improve our description of such molecules. To achieve this goal, a new program package LONDON will be developed to enable all the important standard methods of quantum chemistry to be applied in the presence of magnetic fields, in a strictly gauge-origin independent manner. Using this program, the first high-lev el many-electron studies of atoms and molecules in strong magnetic fields will be performed. With the help of this program, important questions concerning electrons in magnetic fields will be answered.
First, the role of electron correlation will be stud ied. Static correlation will be treated using multi-configurational self-consistent field theory; it will be important for studying (avoided) crossings of electronic states at different field strengths and for describing bond dissociation in strong magnet ic fields. The purported decreasing role of dynamical correlation will be studied using perturbation theory, possibly in combination with self-consistent field theory. The role of relativity in strong magnetic fields will be studied at the four-component Dirac-Hartree-Fock level of theory. Unlike dynamical correlation, the importance of relativity is expected to increase with increasing field strength but no calculations have yet been performed to explore and quantify the increasing importance of relativi ty in strong magnetic fields.
From accurate many-electron wave functions, a variety of molecular properties will be calculated in the MOLMAG project. Of particular interest are properties related to geometrical distortions such as molecular forces and fo rce constant, needed to determine the rotational and vibrational levels of molecules in strong magnetic fields. Finally, electronic excitations will be studied using response theory, properly generalized to gauge-invariant calculations in magnetic fields.