The
third law of thermodynamics is sometimes stated as follows, regarding the properties of systems in equilibrium at
absolute zero temperature: At absolute zero (zero
kelvin), the system must be in a state with the minimum possible energy, and the above statement of the third law holds true provided that the perfect crystal has only one
minimum energy state. Entropy is related to the number of accessible
microstates, and for a system consisting of many particles, quantum mechanics indicates that there is only one unique state (called the
ground state) with minimum energy. If the system does not have a well-defined order (if its order is glassy, for example), then in practice there will remain some finite entropy as the system is brought to very low temperatures as the system becomes locked into a configuration with non-minimal energy. The constant value is called the
residual entropy of the system.