According to the guardaparque in San Martin de los Andes, the Argentine side of the mountain is seasonally closed above 3000m due to crevasse danger. Local guides warn of one large crevasse above the refugios and one smaller crevasse near the summit.
According to CONAF in Pucon (Chile), the Chilean side of the mountain is open all year. In spring and early summer, this route would make a fun snow climb and ski descent. In late summer and fall, the route was mostly snow-free with considerable danger of rockfall. A winter attempt seems feasible, but be weary of storms and avalanche danger.
The route climbs from ~1000m to >3700m, but can be completed in one 10-15 hour day. If electing to spend a night on the mountain, the last halfway decent bivy sites we found were just below 2000m.
Access this route from the highway connecting Junin de los Andes, Argentina and Pucon, Chile. If coming from Argentina, you will not be permitted to bring fresh meats and produce across the border into Chile.
From the Chilean border post, proceed ~2 km west along the gravel highway to a spur road heading south towards the mountain. Follow this road for ~2 km to its end near a large rock pit.
It is possible to camp in the trees near the rockpit, but nicer sites are available by the creek if you walk five minutes further uphill.
From the rockpit, follow a faint trail toward the mountain. As the trail peters out, follow the creek upstream towards a waterfall at the base of the mountain. Climb up the hill and cross over to the east side of the creek right above the falls.
From here, scramble up the rocky ridge on the east side of the drainage. Bear to your left as you climb to find easier scrambling and to avoid unnecessary ups and downs. Eventually work your way back towards the drainage to find some sheltered bivy sites on the east side of the ridge just below 2000m.
From here, climb straight up via the path of your choice, bearing left if uncertain. Generally, the wide ridges are your best bet, but this may vary depending on snow coverage and your equipment selection. At ~3000m, the terrain levels off briefly and your final challenge comes into focus.
Rockfall is especially precarious for the next 500m. Stick to the prominent ridge in the middle to avoid the bowling alleys on either side. Scramble up another steep section until the slope again levels off as you approach the summit.
Beware of a small crevasse (<1 m wide) that you must cross as you approach the summit. In February 2008, the crevasse was obvious and marked with a bamboo wand. The summit itself is relatively flat and appears to be safe, but do not stray too far without extreme caution, as the glacier complexifies quickly.
Ice axe and crampons are necessary in the spring and early summer. Wear a helmet, especially as rockfall danger increases in late summer and early fall. Skis would be fun early in the season with sufficient snow cover.