Coupled-Cluster Valence Bond Theory for Exact Treatment of Spin-Fluctuation

We present a valence bond approach for treating spin-fluctuation called coupled-cluster valence bond (CCVB). In particular, we will discuss the three-pair extension of CCVB, CCVB+i3, and its application to spin-frustrated systems such as molecular magnets. CCVB+i3 can describe spin-fluctuation exactly without invoking neither an exponential wall nor spin contamination. We also discuss a future direction on including charge-fluctuation into this model.