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                 Water slowing down drives the occurrence of the low temperature dynamical transition in microgels                                  

                  Letizia Tavagnacco, Marco Zanatta, Elena Buratti, Monica Bertoldo, Ester Chiessi, Markus Appel,  
                                    Francesca Natali, Andrea Orecchini, and Emanuela Zaccarelli                        

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Atomistic molecular dynamics simulations of PNIPAM microgels suspensions were performed using a nanoscale network model, which is designed from 12 atactic chains connected by 6 BIS cross-links. The atomic coordinates of a network model with a PNIPAM mass fraction 60 % (w/w) are reported in 'network_model_60.pdb'. The OPLS-AA force field with the implementation by Siu et al. is used to describe PNIPAM, while water is modelled with the Tip4p/ICE force field. Simulations were carried out using the GROMACS 2020.6 software.

Simulations were performed in the range between 293 K and 173 K, every 5 K. Equilibration was first carried out at 293 K in a pressure bath at 1 bar up to a constant density value, i.e. tot-drift lower than 2x10^-3 g cm^-3 over 20 ns. A similar equilibration protocol was applied at each temperature explored. Trajectory data were collected in the NVT ensemble for 0.5 micorseconds, with a sampling of 0.2 frame/ps. 
Pressure was controlled by the Parrinello-Rahman algorithm with time constant of 2 ps. Temperature was controlled with the velocity rescaling thermostat coupling algorithm with a time constant of 0.1 ps. Electrostatic interactions were treated with the smooth particle-mesh Ewald method with a cutoff of non-bonded interactions of 1 nm. The leapfrog integration algorithm was employed with a time step of 2 fs. The length of bonds involving hydrogen atoms was kept fixed using the LINCS algorithm. Cubic periodic boundary conditions and minimum image convention were applied.

Molecular dynamics simulations were also carried out for a cubic box containing 1782 Tip4p/ICE water molecules following a similar procedure. 

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