We study the zero-temperature quantum phase transition between liquid and hcp solid 4 He. We use the variational method with a simple yet exchange-symmetric and fully explicit wave function. It is found that the optimized wave function undergoes spontaneous symmetry breaking and describes the quantum solidification of helium at 22 atm. The explicit form of the wave function allows us to consider various contributions to the phase transition. We find that the employed wave function is an excellent candidate for describing both a first-order quantum phase transition and the ground state of a Bose solid. Properties of solid 4 He have regained attention due to a host of unexpected physics discovered in the past decade [1][2][3][4][5][6][7][8]. Most of the new features occur close to absolute zero and are believed to be primarily driven by quantum effects. Consequently, the solidification of 4 He came under renewed scrutiny. The role of quantum statistics in the transition location has been recently revisited in Ref. [9]. At small but nonzero temperatures, indistinguishability of particles destabilizes the quantum solid. Distinguishable particles, on the other hand, would solidify even at low pressures, with the phase diagram reminiscent of the Pomeranchuk effect [10,11]. The feature was dubbed in [9] as thermocrystallization. A similar effect was seen numerically for the Wigner-crystallization of a two-dimensional Coulomb system [12]. The solidification of 4 He at zero temperature was revisited in Ref. [13] with the density functional theory (DFT). Results were improved compared with previous DFT studies.In this paper, we show that the quantum solidification of 4 He can be considered variationally, with a single explicit wave-function which selects the phase through optimization of the thermodynamic potential. Quite surprisingly, we find that the phase transition is predicted properly, given the relative simplicity of the wave function. While the variational treatment is used for quantum phase transitions at the mean-field level [14], it is relatively uncommon that a (discontinuous) transition can be described with a microscopic wave function. The finding is also interesting in light of growing interest to the first-order quantum phase transitions [15,16].At zero temperature, the phases of 4 He can be studied in an essentially exact form with a family of projector methods, including Green's-function Monte Carlo [17,18] [23,24], and describe the transition [25][26][27] and coexistence [28] between the two phases. The SWFs can be seen as representing a single step of a projection calculation [23]. The projection is carried out by performing the numerical integration of the shadow degrees of freedom. In this sense, the SWF is not fully explicit, as one cannot write down the result of such an integration. We consider SWF calculations as a class of their own, in between the exact projection methods and the simple and fully explicit wave function used here.Highly effective wave functions have been developed over the years...