We have performed first-principle calculations of the electronic structure and exchange couplings for the nanotube compound Na2V3O7 using the LDA+U approach. Our results show that while the intra-ring exchange interactions are mainly antiferromagnetic, the inter-ring couplings are {\it ferromagnetic}. We argue that this is a consequence of the strong hybridization between filled and vacant 3d vanadium orbitals due to the low symmetry of Na2V3O7, which results into strong - and often dominant - ferromagnetic contributions to the total exchange interaction between vanadium atoms. A comparison with results of previous works is included.
@article{arxiv.cond-mat/0509315,
title = {Electronic Structure and Exchange Interactions of Na$_{2}$V$_{3}$O$_{7}$},
author = {V. V. Mazurenko and F. Mila and V. I. Anisimov},
journal= {arXiv preprint arXiv:cond-mat/0509315},
year = {2009}
}