Accurate calculations of the helium atom in magnetic fields

The 1¹0⁺, 1¹(-1)⁺and 1¹(-2)⁺ states of the helium atom in the magnetic field regime between 0 and 100 a.u. are studied using a full configuration-interaction (CI) approach. The total energies, derivatives of the total energy with respect to the magnetic field and ionisation energies are calculated with Hylleraas-like functions in spherical coordinates in low to intermediate fields and Hylleraas--Gaussian functions in cylindrical coordinates in intermediate to high fields, respectively. In intermediate fields, the total energies and ionisation energies are determined in terms of Hermite interpolation, based on the results obtained with the two above-mentioned basis functions. Calculations show that the current method can produce lower total energies and larger ionisation energies, and make the two ionisation energy curves obtained with the two above-mentioned basis functions join smoothly in intermediate fields. Comparisons are also made with previous works.