Optimism is a term seldom used when describing the prospect of a nuclear deal with Iran.
Indeed nobody expects dramatic breakthroughs when Iran sits down on Tuesday in Geneva with six world powers for talks aimed at curbing its nuclear ambitions.
But a new feel-good tone from Tehran toward the United States and its western allies since the election of President Hassan Rouhani has diplomats hoping that an agreement might be possible after a decade-long standoff.
"The stakes are higher because the expectations are so much higher," one senior U.S. official said. "We aren't naive about the challenges but there is a new openness that is encouraging."
During his visit to the United Nations General Assembly in September, Rouhani's diplomatic approach raised hopes in the West of a thaw with Iran and progress in negotiations on its nuclear program.
Western powers believe Iran is trying to develop a nuclear weapon, but Tehran has long insisted that its intentions are peaceful.
Rouhani's visit culminated in a phone call with President Barack Obama and a meeting between Secretary of State John Kerry and Iranian Foreign Minister Javad Zarif.
It was the first such high-level contact between the two sworn enemies since Iran's 1979 revolution, which sent relations between the two into a deep freeze.
'Mistrust' colors chances for breakthrough
Despite the improved climate, a senior Obama administration official told reporters in Geneva that Washington is "not expecting a breakthrough overnight," given the history of mistrust between Iran and the West and the myriad of technical issues involved.
But the official hoped both sides can agree on a set of measures while they negotiate a comprehensive agreement.
The official added Iran previously used the time for negotiations to continue developing its nuclear program, but added "we cannot continue for that to be the case."
"There needs to be a confidence-building step that not only starts to get over the deep mistrust ... but also constrains Iran's program today and perhaps even takes it back a notch so that there is time on the clock to achieve that comprehensive agreement," the official said.
Rouhani, who replaced conservative hard-liner Mahmoud Ahmadinejad, was elected with a mandate to end the standoff and lift international sanctions that have nearly crippled its oil exports and access the international financial system.
The senior official briefing reporters said Washington wants to keep up that economic pressure as negotiations get under way.
"We believe that has got us to where we are today," the official said.
But the official said the Obama administration would be willing to consider quick relief on sanctions "targeted in proportion" to what Iran puts on the table should it be prepared to curtail the pace and scope of its uranium enrichment program, offer steps to improve transparency of its nuclear program, and address concerns over its stockpile of enriched uranium.
The official noted that sanctions experts from the State and Treasury departments are part of the American delegation in Geneva for the talks.
"If Iran is ready to go, we are ready to go," the official said.
'No one is naive'
Iran has signalled in recent days that it is ready to deal, with Zarif and his deputy, both saying the delegation is bringing a new proposal to the table.
As he headed to Geneva, Zarif repeated Iran's desire to reach an agreement.
"Tomorrow is the start of a difficult and relatively time-consuming way forward," the U.S.-educated veteran diplomat wrote in a post on his Facebook page late Sunday. "I am hopeful that by Wednesday we can reach agreement on a road map to find a path towards resolution."
The bloc of countries leading the diplomatic effort include five permanent U.N. Security Council members -- the United States, Russia, China, France and Britain -- plus Germany.
When Zarif met with the group last month, he presented what one senior U.S. official called "the most forward-leaning ideas" the Iranians have ever offered to dispel international concerns over its nuclear program since diplomacy began over a decade ago.