I'm trying to simulate the complex [V(H2O)6]3+ using Guassian, but the calculation does not converge.
This is a convergence issue which can be due to a variety of reasons. My advice is to reformulate your z-matrix (avoid Cartesian coords in possible) so that the starting geometry has the highest possible symmetry, i.e. belongs to a point group with the largest number of symmetry elements (which is probably Th or -worse- D2h). Second, start with a simple basis set (def2svp) and increase its complexity only if needed. Third, try different convergence algorithms (for example a slower quadratic one, qc in Gaussian).
Finally, and maybe most importantly, there is already at least a paper on this topic, dealing exactly [V(OH2)6]3+. Give it a glance:
Article Assessment of TD-DFT and LF-DFT for study of d - D transitio...
Thank you very much for your answer and thank you for the article. I will try your suggestions.