DFT+U methods

OpenMX supports various types of DFT+U methods with respect to the treatment of the occupation number operator, the functional form, and the choice of the double counting term. The functionality is supported for both the collinear and non-collinear calculations. To acknowledge in any publications by using the functionality, the citation of the references [20,21,22] would be appreciated. The technical details of the methods and its implementation can be found in Ref. [20,21,22] and technical notes [23,24].