Theoretical study of the nitrogen atom hyperfine coupling constant

The nitrogen atom isotropic hyperfine coupling constant A/sub iso/ is studied as a function of improvements in both the one-particle and n-particle basis sets. Our study underscores the importance of diffuse basis functions. For example, the (9s 5p) primitive set of Huzinaga augmented with an even-tempered diffuse s function yields values for A/sub iso/ that are virtually identical to an energy-optimized (23s 12p) even-tempered set. The A/sub iso/ constant is found to converge relatively quickly with increasing l quantum numbers: d, f, and g functions are estimated to contribute 2.5 +- 0.2, 0.4 +- 0.1, and 0.05 +- 0.05 MHz, respectively. Full CI calibration calculations indicate that very high levels of correlation treatment are required for quantitative results. In addition, a strong coupling is observed between the one-particle and n-particle requirements. Our best result of 10.4 MHz is in excellent agreement with the accurate experimental value of 10.4509 MHz