CONSTRUCTION OF THE ADIABATIC CONNECTION

Two models for the change in the lambda-dependent exchange-correlation energy upon atomization Delta E(xc,lambda) = E(xc,lambda)(atoms) - E( xc,lambda)(molecule) are proposed, where E(sc,lambda) = [psi(lambda)\< (V)over cap (ee)>)\psi(lambda)] - integral d(3)r d(3)r' rho(r)rho(r')/ 2\r - r'\. The wavefunction psi(lambda) yields the ground-state densit y rho and minimizes (T) over cap + <lambda(V)over cap (ee)>. These mod els (Delta E(xc,lambda)(model)) make use of the exact E(x) and general ized gradient approximations (GGAs) to E(xc). The construction of the simplest model is verified by calculating the exact d Delta E(xc,lambd a)/d lambda\(lambda=0) from density functional perturbation theory and comparing it to d Delta E(xc,lambda)(model)/d lambda\(lambda=0). For systems with strong static correlation, explicit inclusion of d Delta E(xc,lambda)/d lambda\(lambda=0) further improves the approximation to Delta E(xc,lambda). Atomization energies calculated from Delta E(xc,l ambda)(model) show a significant improvement over GGA.