Quantum computational chemistry has emerged as a potential application of  quantum computing. Hybrid quantum-classical computing methods, such as  variational quantum eigensolvers, have been designed as promising solutions  to quantum chemistry problems. Nonetheless, challenges due to theoretical  complexity and experimental imperfections hinder progress in achieving  reliable and accurate results. Experimental works for solving electronic  structures are consequently still restricted to non-scalable or classically  simulable ansatz or limited to a few qubits with large errors. Here, we address  the critical challenges associated with solving molecular electronic structures  using noisy quantum processors. Our protocol presents improvements in the  circuit depth and running time, key metrics for chemistry simulation. Through  systematic hardware enhancements and the integration of error-mitigation  techniques, we overcome theoretical and experimental limitations and  successfully scale up the implementation of variational quantum eigensolvers  with an optimized unitary coupled cluster ansatz to 12 qubits. We produce  high-precision results of the ground-state energy for molecules with error  suppression by around two orders of magnitude. Our work demonstrates a  feasible path towards a scalable solution to electronic structure calculation.