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## Spin-free Exact Two-component Theory In Its One-electron Variant Sf X 2 C-1eSpin-free exact two-component theory in its one-electron variant (SFX2C-1e) offers accurate and efficient (essentially as efficient as the non-relativistic case) treatments of scalar-relativistic effects by a simple modification of the one-electron Hamiltonian matrix elements, and thus is recommended for scalar-relativistic calculations. A SFX2C-1e calculation can be performed with RELATIVISTIC=X2C1E Both energy and first derivatives (currently restricted to dipole moment, electric-field gradients, and geometrical gradients) calculations can be carried out. It is compatible with all the quantum-chemical methods to treat electron correlation implemented in CFOUR and also AO-based algorithm to enhance efficiency. One may use SFX2C-1e recontracted basis sets or uncontracted basis sets. Examples: SFX2C-1e single point energy calculation SFX2C-1e geometry optimization X2C basis functions: a) The correlation-consistent basis sets have been recontracted for the SFX2C-1e method (under construction; more basis sets and documentation to come). Recontracted correlation-consistent basis functions b) ANO basis sets obtained by recontracting the ANO-RCC primitive sets using the SFX2C-1e-CCSD density together with Gaussian nuclear model (under construction; more basis sets and documentation to come). |

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CFOUR is partially supported by the U.S. National Science Foundation.