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# During years research efforts at Los

During 2002–2004years research efforts at Los Alamos National Laboratory (LANL) focused on constructing a quantum computer based on regular arrays build from neutral radical molecules possessing one single unpaired SU 4312 spin. The idea was built on the ability to manipulate individual electron spins in some kind of a solid matrix or lattice [5], [6], [7]. It was suggested that self-assembled monolayer systems could be used to create a macroscopic ensemble of quantum entangled 3-spin groups as a first step in quantum information processing [8], [9]. The spins of such a group could be connected by dipole–dipole quantum couplings. Application of a non-uniform external magnetic field could allow selective excitation of every spin inside the group. The proper sequence of resonant electromagnetic pulses would then drive all spin groups into a 3-spin entangled state. In the approach suggested in Ref. [5], the spins were to be associated with a single unpaired electron spin of a neutral radical molecule in the self-assembled monolayer.
One of the key elements of this strategy is the proper choice of molecules for experimental implementation of quantum computing in liquid systems. Involved in this choice are criteria for the stability of chemical structures of these molecules [1].

Procedures/methodologies that have been used
The quantum mechanical electron correlation interaction density functional theory (DFT) B97d/SVP method/basis set with Grimme long-range dispersion electron corrections [10], [11] (i.e., high precision quantum mechanical simulations) were used to investigate ACh molecule and ACh molecular complexes in water molecules environment in various spin multiplicity states.
The quantum simulations of single bioorganic molecules or their complexes possessing closed or open electronic shells start from an arbitrary geometry (Cartesian coordinates of the nuclei). Using quantum mechanical semi-empirical PM6 and DFT B97d approaches in the Gaussian09 [11] program package, we obtain the lowest molecular energy that parametrically depends on the coordinates of the nuclei. These coordinates are adjusted using the standard geometry optimization procedure to minimize the energy with respect to the nuclear positions. Special care is required to verify that the obtained optimal molecular structure is a global minimum in the phase space of the nuclear (3n-6, n being the number of atoms) degrees of freedom.

Electron spin density localization in complexes of ACh molecules in water environment
The neutral radical ACh molecule is not regular, because its nitrogen atom (colored by light blue in Fig. 1) possesses four chemical bonds with carbon atoms, even though it typically forms only 3 single bonds. Therefore ACh molecule possesses not compensated spin. The geometry of the single acetylcholine neutral radical molecule surrounded by 20 water molecules was determined by finding the electronic configuration with multiplicity equal to 2 that minimized its energy (see Fig 1). This was done using DFT with the unrestricted B97d/SVP potential/basis set and calculated in Gaussian09 program package.
The results of atomic electron spin densities calculated using the Gaussian09 unrestricted B97d/SVP potential/basis of hydrated ACh neutral radical molecule possesses atomic spatially localized electronic spin density on the acetate (C2H3O2−) group equal to 0.973 (see Table 1). The net spin of the molecule is normalized to one unit.
One of ACh hydration results was that spin density localization shifted from -NH4 to acetate group (compare with results of Ref. [1]) but spin density still localized in relatively small area that allow to manipulate with this spin by external magnetic field or control this spin by the choosing of proper parameters of g-tensor of attached molecules to ACh complex as it have been done in previous research, see Ref. [9], [12].
It is described in Ref. [1] how we obtained structure of two ACh molecules in vacuum. Having results of ACh dimer optimization we have scaled the system of ACh molecules in our present article investigating one, two, three, four and five ACh molecules in water molecules environment and optimizing geometries of these complexes by DFT B97d/SVP method/basis set method.