The paper explores the performance of numerical atomic orbital (NAO) basis sets in density-functional calculations of solids and molecules, aiming to optimize basis quality while maintaining strict localization of orbitals for linear-scaling calculations. Several schemes are tested, and the best performance is achieved with a new scheme proposed in this work, which is a flexibilization of previous proposals. The basis sets are tested against converged plane-wave calculations on a variety of systems, including covalent, ionic, and metallic materials. Satisfactory convergence is obtained for reasonably small basis sizes, with clear improvements over previous schemes. The transferability of the obtained basis sets is also tested and found to be satisfactory. The paper discusses the optimization procedure, basis convergence, transferability, and the effect of tightening the orbital range. The results show that the proposed NAO basis sets offer substantial improvements in accuracy and efficiency compared to previous schemes, with good transferability and the ability to achieve high precision with modest basis sizes.The paper explores the performance of numerical atomic orbital (NAO) basis sets in density-functional calculations of solids and molecules, aiming to optimize basis quality while maintaining strict localization of orbitals for linear-scaling calculations. Several schemes are tested, and the best performance is achieved with a new scheme proposed in this work, which is a flexibilization of previous proposals. The basis sets are tested against converged plane-wave calculations on a variety of systems, including covalent, ionic, and metallic materials. Satisfactory convergence is obtained for reasonably small basis sizes, with clear improvements over previous schemes. The transferability of the obtained basis sets is also tested and found to be satisfactory. The paper discusses the optimization procedure, basis convergence, transferability, and the effect of tightening the orbital range. The results show that the proposed NAO basis sets offer substantial improvements in accuracy and efficiency compared to previous schemes, with good transferability and the ability to achieve high precision with modest basis sizes.