Subsystem constraints in variational second order density matrix optimization: curing the dissociative behavior.

Area of Science:

• Quantum chemistry
• Computational physics
• Theoretical chemistry

Background:

• Variational second-order density matrix theory faces challenges in the dissociation limit for diatomic molecules.
• Standard N-representability conditions (two-index P,Q,G or three-index T(1),T(2)) lead to fractional atomic charges upon dissociation.
• Accurate description of molecular dissociation is crucial for understanding chemical reactions and properties.

Purpose of the Study:

• To introduce a novel class of N-representability conditions, termed subsystem constraints.
• To demonstrate that these subsystem constraints resolve the dissociation problem in variational second-order density matrix theory.
• To assess the computational cost and applicability of the new method.

Main Methods:

• Development of a general framework for subsystem constraints.
• Application of subsystem constraints to variational second-order density matrix theory.
• Numerical study of the singlet potential energy surface for the heteronuclear molecule Be B(+) as a test case.

Main Results:

• Subsystem constraints successfully eliminate fractional charges at the dissociation limit.
• The proposed method maintains accuracy while incurring minimal additional computational expense.
• The singlet potential energy surface of Be B(+) is accurately reproduced.

Conclusions:

• Subsystem constraints offer a robust solution to the dissociation problem in density matrix theory.
• The method is computationally efficient and applicable to both diatomic and polyatomic molecules.
• This advancement facilitates more reliable quantum chemical calculations.