Using aug-cc-pVDZ basis sets supplemented with diffused bond functions, the (H2O)(4)(-) isomer of the water tetramer anion in Ci-symmetry was optimized at the MP2 level. The excess electron was enveloped directly by the four hydrogen atoms of two inside water molecules, forming an interior structure. This study shows four hydrogen atoms of two outside water molecules also have important interaction with the trapped electron. We described the interactions between individual water molecule (including both inside and outside) and the trapped electron. The influence of the trapped electron on the surrounded second layer molecules was also put forward. (C) 2003 Elsevier Science B.V. All rights reserved.