To elucidate microscopic details of proton cancer therapy (PCT), we apply the 
simplest-level electron nuclear dynamics (SLEND) method to H+ + (H2O)1-6 at ELab 
= 100 keV. These systems are computationally tractable prototypes to simulate 
water radiolysis reactions-i.e. the PCT processes that generate the DNA-damaging 
species against cancerous cells. To capture incipient bulk-water effects, ten 
(H2O)1-6 isomers are considered, ranging from quasi-planar/multiplanar (H2O)1-6 
to "smallest-drop" prism and cage (H2O)6 structures. SLEND is a time-dependent, 
variational, non-adiabatic and direct method that adopts a nuclear 
classical-mechanics description and an electronic single-determinantal 
wavefunction in the Thouless representation. Short-time SLEND/6-31G* (n = 1-6) 
and /6-31G** (n = 1-5) simulations render cluster-to-projectile 
1-electron-transfer (1-ET) total integral cross sections (ICSs) and 1-ET 
probabilities. In absolute quantitative terms, SLEND/6-31G* 1-ET ICS compares 
satisfactorily with alternative experimental and theoretical results only 
available for n = 1 and exhibits almost the same accuracy of the best 
alternative theoretical result. SLEND/6-31G** overestimates 1-ET ICS for n = 1, 
but a comparable overestimation is also observed with another theoretical 
method. An investigation on H+ + H indicates that electron direct ionization 
(DI) becomes significant with the large virtual-space quasi-continuum in large 
basis sets; thus, SLEND/6-31G** 1-ET ICS is overestimated by DI contributions. 
The solution to this problem is discussed. In relative quantitative terms, both 
SLEND/6-31* and /6-31G** 1-ET ICSs precisely fit into physically justified 
scaling formulae as a function of the cluster size; this indicates SLEND's 
suitability for predicting properties of water clusters with varying size. 
Long-time SLEND/6-31G* (n = 1-4) simulations predict the formation of the 
DNA-damaging radicals H, OH, O and H3O. While "smallest-drop" isomers are 
included, no early manifestations of bulk water PCT properties are observed and 
simulations with larger water clusters will be needed to capture those effects. 
This study is the largest SLEND investigation on water radiolysis to date.