A set of basis functions employed for the representation of molecular orbitals. One may distinguish the minimal basis set (includes one @BT06998@ for each SCF occupied atomic orbital with distinct principal and @A00353@ quantum numbers); split @V06588@ basis set (includes two or more sizes of @BT06998@ for each @V06588@ orbital); double @Z06745@ (DZ) basis set (a split @V06588@ basis set that includes exactly twice as many functions as the minimal basis set); extended basis set (the set larger than the double @Z06745@ basis set); polarized basis set (incorporates basis functions of higher angular quantum number beyond what is required by the atom in its electronic @G02704@; allows orbitals to change not only a size, but also a shape); basis set with diffuse functions and others.