PC Spartan uses mathematical methods which perform molecular mechanics calculations using SYBYL force field, AM1, PM3, and AM1-SM2 semi-emperical orbital calculations. Although these calculations are not as proficient as ab initio calculations, they are good for fast approximations. A point to note is that for calculations involving transition metals, it is best to use the PM3 algorithm.
Also PC Spartan conducts ab initio calculations using 3-21G(*) and 6-31G(*) basis sets. Ab initio represents first principle calculations, meaning the nature of the calculations are quantum mechanical in origin. Where these calculations are indeed the closest to the actual theoretical value, they are very time consuming. It is often best to use a semi-emperical method, which combines molecular mechanics and ab initio calculations.