Multiparameter equations of state are empirical equations of state that can be used to represent pure fluids with high accuracy. Multiparameter equations of state are empirical correlations of experimental data and are usually formulated in the Helmholtz free energy. The functional form of these models is in most parts not physically motivated. They can be usually applied in both liquid and gaseous states. Empirical multiparameter equations of state represent the Helmholtz energy of the fluid as the sum of ideal gas and residual terms. Both terms are explicit in temperature and density: a ( T , ρ ρ ) R T = a i d e a l g a s ( τ τ , δ δ ) + a residual ( τ τ , δ δ ) R T with τ τ = T r T , δ δ = ρ ρ ρ ρ r