Explain the architecture and workflow of a Variational Quantum Eigensolver (VQE) algorithm applied to finding the ground state energy of a simple diatomic molecule (e.g., H2). Detail the role of the Ansatz (parameterized quantum circuit) and the classical optimizer in the hybrid quantum-classical loop. Discuss the challenges of 'barren plateaus' in the optimization landscape and how noise in current NISQ (Noisy Intermediate-Scale Quantum) devices affects the convergence of the algorithm. Propose a method for error mitigation to improve the accuracy of the energy calculation.