网上看到的资料“Two new keywords have been introduced to deal with the fairly common situation, for excited states, that weakly coupled intruders cause spurious singularities, `spikes' in e.g. a potential curve. The two new keywords SHIFT and IMAGINARY SHIFT (mutually exclusive) will introduce a shift in the energy denominators, thus avoiding singularities, and will also correct the energy for the use of this shift. The net effect is that the energy is almost unaffected except in the vicinity of the weak singularity, which is removed. The SHIFT keyword adds a real shift, and the use of this procedure is well tested [94,96]. The IMAGINARY SHIFT adds an imaginary quantity, and then uses the real value of the resulting second-order energy [40]. This offers some advantage, but is not well tested yet.” http://vergil.chemistry.gatech.e ... exclusive%29%20will,the%20vicinity%20of%20the%20weak%20singularity%2C%20which%20is